ginac/clifford.cpp

   1 /** @file clifford.cpp
   2  *
   3  *  Implementation of GiNaC's clifford algebra (Dirac gamma) objects. */
   4
   5 /*
   6  *  GiNaC Copyright (C) 1999-2001 Johannes Gutenberg University Mainz, Germany
   7  *
   8  *  This program is free software; you can redistribute it and/or modify
   9  *  it under the terms of the GNU General Public License as published by
  10  *  the Free Software Foundation; either version 2 of the License, or
  11  *  (at your option) any later version.
  12  *
  13  *  This program is distributed in the hope that it will be useful,
  14  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  15  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  16  *  GNU General Public License for more details.
  17  *
  18  *  You should have received a copy of the GNU General Public License
  19  *  along with this program; if not, write to the Free Software
  20  *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  21  */
  22
  23 #include "clifford.h"
  24 #include "ex.h"
  25 #include "idx.h"
  26 #include "ncmul.h"
  27 #include "symbol.h"
  28 #include "numeric.h" // for I
  29 #include "symmetry.h"
  30 #include "lst.h"
  31 #include "relational.h"
  32 #include "print.h"
  33 #include "archive.h"
  34 #include "debugmsg.h"
  35 #include "utils.h"
  36
  37 #include <stdexcept>
  38
  39 namespace GiNaC {
  40
  41 GINAC_IMPLEMENT_REGISTERED_CLASS(clifford, indexed)
  42 GINAC_IMPLEMENT_REGISTERED_CLASS(diracone, tensor)
  43 GINAC_IMPLEMENT_REGISTERED_CLASS(diracgamma, tensor)
  44 GINAC_IMPLEMENT_REGISTERED_CLASS(diracgamma5, tensor)
  45
  46 //////////
  47 // default constructor, destructor, copy constructor assignment operator and helpers
  48 //////////
  49
  50 clifford::clifford() : representation_label(0)
  51 {
  52         debugmsg("clifford default constructor", LOGLEVEL_CONSTRUCT);
  53         tinfo_key = TINFO_clifford;
  54 }
  55
  56 void clifford::copy(const clifford & other)
  57 {
  58         inherited::copy(other);
  59         representation_label = other.representation_label;
  60 }
  61
  62 DEFAULT_DESTROY(clifford)
  63 DEFAULT_CTORS(diracone)
  64 DEFAULT_CTORS(diracgamma)
  65 DEFAULT_CTORS(diracgamma5)
  66
  67 //////////
  68 // other constructors
  69 //////////
  70
  71 /** Construct object without any indices. This constructor is for internal
  72  *  use only. Use the dirac_ONE() function instead.
  73  *  @see dirac_ONE */
  74 clifford::clifford(const ex & b, unsigned char rl) : inherited(b), representation_label(rl)
  75 {
  76         debugmsg("clifford constructor from ex", LOGLEVEL_CONSTRUCT);
  77         tinfo_key = TINFO_clifford;
  78 }
  79
  80 /** Construct object with one Lorentz index. This constructor is for internal
  81  *  use only. Use the dirac_gamma() function instead.
  82  *  @see dirac_gamma */
  83 clifford::clifford(const ex & b, const ex & mu, unsigned char rl) : inherited(b, mu), representation_label(rl)
  84 {
  85         debugmsg("clifford constructor from ex,ex", LOGLEVEL_CONSTRUCT);
  86         GINAC_ASSERT(is_ex_of_type(mu, varidx));
  87         tinfo_key = TINFO_clifford;
  88 }
  89
  90 clifford::clifford(unsigned char rl, const exvector & v, bool discardable) : inherited(sy_none(), v, discardable), representation_label(rl)
  91 {
  92         debugmsg("clifford constructor from unsigned char,exvector", LOGLEVEL_CONSTRUCT);
  93         tinfo_key = TINFO_clifford;
  94 }
  95
  96 clifford::clifford(unsigned char rl, exvector * vp) : inherited(sy_none(), vp), representation_label(rl)
  97 {
  98         debugmsg("clifford constructor from unsigned char,exvector *", LOGLEVEL_CONSTRUCT);
  99         tinfo_key = TINFO_clifford;
 100 }
 101
 102 //////////
 103 // archiving
 104 //////////
 105
 106 clifford::clifford(const archive_node &n, const lst &sym_lst) : inherited(n, sym_lst)
 107 {
 108         debugmsg("clifford constructor from archive_node", LOGLEVEL_CONSTRUCT);
 109         unsigned rl;
 110         n.find_unsigned("label", rl);
 111         representation_label = rl;
 112 }
 113
 114 void clifford::archive(archive_node &n) const
 115 {
 116         inherited::archive(n);
 117         n.add_unsigned("label", representation_label);
 118 }
 119
 120 DEFAULT_UNARCHIVE(clifford)
 121 DEFAULT_ARCHIVING(diracone)
 122 DEFAULT_ARCHIVING(diracgamma)
 123 DEFAULT_ARCHIVING(diracgamma5)
 124
 125 //////////
 126 // functions overriding virtual functions from base classes
 127 //////////
 128
 129 int clifford::compare_same_type(const basic & other) const
 130 {
 131         GINAC_ASSERT(is_of_type(other, clifford));
 132         const clifford &o = static_cast<const clifford &>(other);
 133
 134         if (representation_label != o.representation_label) {
 135                 // different representation label
 136                 return representation_label < o.representation_label ? -1 : 1;
 137         }
 138
 139         return inherited::compare_same_type(other);
 140 }
 141
 142 bool clifford::match_same_type(const basic & other) const
 143 {
 144         GINAC_ASSERT(is_of_type(other, clifford));
 145         const clifford &o = static_cast<const clifford &>(other);
 146
 147         return representation_label == o.representation_label;
 148 }
 149
 150 DEFAULT_COMPARE(diracone)
 151 DEFAULT_COMPARE(diracgamma)
 152 DEFAULT_COMPARE(diracgamma5)
 153
 154 DEFAULT_PRINT_LATEX(diracone, "ONE", "\\mathbb{1}")
 155 DEFAULT_PRINT_LATEX(diracgamma, "gamma", "\\gamma")
 156 DEFAULT_PRINT_LATEX(diracgamma5, "gamma5", "{\\gamma^5}")
 157
 158 /** Contraction of a gamma matrix with something else. */
 159 bool diracgamma::contract_with(exvector::iterator self, exvector::iterator other, exvector & v) const
 160 {
 161         GINAC_ASSERT(is_ex_of_type(*self, clifford));
 162         GINAC_ASSERT(is_ex_of_type(*other, indexed));
 163         GINAC_ASSERT(is_ex_of_type(self->op(0), diracgamma));
 164         unsigned char rl = ex_to<clifford>(*self).get_representation_label();
 165
 166         if (is_ex_of_type(*other, clifford)) {
 167
 168                 ex dim = ex_to<idx>(self->op(1)).get_dim();
 169
 170                 // gamma~mu gamma.mu = dim ONE
 171                 if (other - self == 1) {
 172                         *self = dim;
 173                         *other = dirac_ONE(rl);
 174                         return true;
 175
 176                 // gamma~mu gamma~alpha gamma.mu = (2-dim) gamma~alpha
 177                 } else if (other - self == 2
 178                         && is_ex_of_type(self[1], clifford)) {
 179                         *self = 2 - dim;
 180                         *other = _ex1();
 181                         return true;
 182
 183                 // gamma~mu gamma~alpha gamma~beta gamma.mu = 4 g~alpha~beta + (dim-4) gamam~alpha gamma~beta
 184                 } else if (other - self == 3
 185                         && is_ex_of_type(self[1], clifford)
 186                         && is_ex_of_type(self[2], clifford)) {
 187                         *self = 4 * lorentz_g(self[1].op(1), self[2].op(1)) * dirac_ONE(rl) + (dim - 4) * self[1] * self[2];
 188                         self[1] = _ex1();
 189                         self[2] = _ex1();
 190                         *other = _ex1();
 191                         return true;
 192
 193                 // gamma~mu S gamma~alpha gamma.mu = 2 gamma~alpha S - gamma~mu S gamma.mu gamma~alpha
 194                 // (commutate contracted indices towards each other, simplify_indexed()
 195                 // will re-expand and re-run the simplification)
 196                 } else {
 197                         exvector::iterator it = self + 1, next_to_last = other - 1;
 198                         while (it != other) {
 199                                 if (!is_ex_of_type(*it, clifford))
 200                                         return false;
 201                                 ++it;
 202                         }
 203
 204                         it = self + 1;
 205                         ex S = _ex1();
 206                         while (it != next_to_last) {
 207                                 S *= *it;
 208                                 *it++ = _ex1();
 209                         }
 210
 211                         *self = 2 * (*next_to_last) * S - (*self) * S * (*other) * (*next_to_last);
 212                         *next_to_last = _ex1();
 213                         *other = _ex1();
 214                         return true;
 215                 }
 216         }
 217
 218         return false;
 219 }
 220
 221 /** Perform automatic simplification on noncommutative product of clifford
 222  *  objects. This removes superfluous ONEs, permutes gamma5's to the front
 223  *  and removes squares of gamma objects. */
 224 ex clifford::simplify_ncmul(const exvector & v) const
 225 {
 226         exvector s;
 227         s.reserve(v.size());
 228
 229         // Remove superfluous ONEs
 230         exvector::const_iterator cit = v.begin(), citend = v.end();
 231         while (cit != citend) {
 232                 if (!is_ex_of_type(cit->op(0), diracone))
 233                         s.push_back(*cit);
 234                 cit++;
 235         }
 236
 237         bool something_changed = false;
 238         int sign = 1;
 239
 240         // Anticommute gamma5's to the front
 241         if (s.size() >= 2) {
 242                 exvector::iterator first = s.begin(), next_to_last = s.end() - 2;
 243                 while (true) {
 244                         exvector::iterator it = next_to_last;
 245                         while (true) {
 246                                 exvector::iterator it2 = it + 1;
 247                                 if (!is_ex_of_type(it->op(0), diracgamma5) && is_ex_of_type(it2->op(0), diracgamma5)) {
 248                                         it->swap(*it2);
 249                                         sign = -sign;
 250                                         something_changed = true;
 251                                 }
 252                                 if (it == first)
 253                                         break;
 254                                 --it;
 255                         }
 256                         if (next_to_last == first)
 257                                 break;
 258                         --next_to_last;
 259                 }
 260         }
 261
 262         // Remove squares of gamma5
 263         while (s.size() >= 2 && is_ex_of_type(s[0].op(0), diracgamma5) && is_ex_of_type(s[1].op(0), diracgamma5)) {
 264                 s.erase(s.begin(), s.begin() + 2);
 265                 something_changed = true;
 266         }
 267
 268         // Remove equal adjacent gammas
 269         if (s.size() >= 2) {
 270                 exvector::iterator it = s.begin(), itend = s.end() - 1;
 271                 while (it != itend) {
 272                         ex & a = it[0];
 273                         ex & b = it[1];
 274                         if (is_ex_of_type(a.op(0), diracgamma) && is_ex_of_type(b.op(0), diracgamma)) {
 275                                 const ex & ia = a.op(1);
 276                                 const ex & ib = b.op(1);
 277                                 if (ia.is_equal(ib)) {
 278                                         a = lorentz_g(ia, ib);
 279                                         b = dirac_ONE(representation_label);
 280                                         something_changed = true;
 281                                 }
 282                         }
 283                         ++it;
 284                 }
 285         }
 286
 287         if (s.empty())
 288                 return clifford(diracone(), representation_label) * sign;
 289         if (something_changed)
 290                 return nonsimplified_ncmul(s) * sign;
 291         else
 292                 return simplified_ncmul(s) * sign;
 293 }
 294
 295 ex clifford::thisexprseq(const exvector & v) const
 296 {
 297         return clifford(representation_label, v);
 298 }
 299
 300 ex clifford::thisexprseq(exvector * vp) const
 301 {
 302         return clifford(representation_label, vp);
 303 }
 304
 305 //////////
 306 // global functions
 307 //////////
 308
 309 ex dirac_ONE(unsigned char rl)
 310 {
 311         return clifford(diracone(), rl);
 312 }
 313
 314 ex dirac_gamma(const ex & mu, unsigned char rl)
 315 {
 316         if (!is_ex_of_type(mu, varidx))
 317                 throw(std::invalid_argument("index of Dirac gamma must be of type varidx"));
 318
 319         return clifford(diracgamma(), mu, rl);
 320 }
 321
 322 ex dirac_gamma5(unsigned char rl)
 323 {
 324         return clifford(diracgamma5(), rl);
 325 }
 326
 327 ex dirac_gamma6(unsigned char rl)
 328 {
 329         return clifford(diracone(), rl) + clifford(diracgamma5(), rl);
 330 }
 331
 332 ex dirac_gamma7(unsigned char rl)
 333 {
 334         return clifford(diracone(), rl) - clifford(diracgamma5(), rl);
 335 }
 336
 337 ex dirac_slash(const ex & e, const ex & dim, unsigned char rl)
 338 {
 339         varidx mu((new symbol)->setflag(status_flags::dynallocated), dim);
 340         return indexed(e, mu.toggle_variance()) * dirac_gamma(mu, rl);
 341 }
 342
 343 /** Check whether a given tinfo key (as returned by return_type_tinfo()
 344  *  is that of a clifford object with the specified representation label. */
 345 static bool is_clifford_tinfo(unsigned ti, unsigned char rl)
 346 {
 347         return ti == (TINFO_clifford + rl);
 348 }
 349
 350 /** Check whether a given tinfo key (as returned by return_type_tinfo()
 351  *  is that of a clifford object (with an arbitrary representation label). */
 352 static bool is_clifford_tinfo(unsigned ti)
 353 {
 354         return (ti & ~0xff) == TINFO_clifford;
 355 }
 356
 357 /** Take trace of a string of an even number of Dirac gammas given a vector
 358  *  of indices. */
 359 static ex trace_string(exvector::const_iterator ix, unsigned num)
 360 {
 361         // Tr gamma.mu gamma.nu = 4 g.mu.nu
 362         if (num == 2)
 363                 return lorentz_g(ix[0], ix[1]);
 364
 365         // Tr gamma.mu gamma.nu gamma.rho gamma.sig = 4 (g.mu.nu g.rho.sig + g.nu.rho g.mu.sig - g.mu.rho g.nu.sig
 366         else if (num == 4)
 367                 return lorentz_g(ix[0], ix[1]) * lorentz_g(ix[2], ix[3])
 368                      + lorentz_g(ix[1], ix[2]) * lorentz_g(ix[0], ix[3])
 369                      - lorentz_g(ix[0], ix[2]) * lorentz_g(ix[1], ix[3]);
 370
 371         // Traces of 6 or more gammas are computed recursively:
 372         // Tr gamma.mu1 gamma.mu2 ... gamma.mun =
 373         //   + g.mu1.mu2 * Tr gamma.mu3 ... gamma.mun
 374         //   - g.mu1.mu3 * Tr gamma.mu2 gamma.mu4 ... gamma.mun
 375         //   + g.mu1.mu4 * Tr gamma.mu3 gamma.mu3 gamma.mu5 ... gamma.mun
 376         //   - ...
 377         //   + g.mu1.mun * Tr gamma.mu2 ... gamma.mu(n-1)
 378         exvector v(num - 2);
 379         int sign = 1;
 380         ex result;
 381         for (unsigned i=1; i<num; i++) {
 382                 for (unsigned n=1, j=0; n<num; n++) {
 383                         if (n == i)
 384                                 continue;
 385                         v[j++] = ix[n];
 386                 }
 387                 result += sign * lorentz_g(ix[0], ix[i]) * trace_string(v.begin(), num-2);
 388                 sign = -sign;
 389         }
 390         return result;
 391 }
 392
 393 ex dirac_trace(const ex & e, unsigned char rl, const ex & trONE)
 394 {
 395         if (is_ex_of_type(e, clifford)) {
 396
 397                 if (ex_to<clifford>(e).get_representation_label() == rl
 398                  && is_ex_of_type(e.op(0), diracone))
 399                         return trONE;
 400                 else
 401                         return _ex0();
 402
 403         } else if (is_ex_exactly_of_type(e, mul)) {
 404
 405                 // Trace of product: pull out non-clifford factors
 406                 ex prod = _ex1();
 407                 for (unsigned i=0; i<e.nops(); i++) {
 408                         const ex &o = e.op(i);
 409                         if (is_clifford_tinfo(o.return_type_tinfo(), rl))
 410                                 prod *= dirac_trace(o, rl, trONE);
 411                         else
 412                                 prod *= o;
 413                 }
 414                 return prod;
 415
 416         } else if (is_ex_exactly_of_type(e, ncmul)) {
 417
 418                 if (!is_clifford_tinfo(e.return_type_tinfo(), rl))
 419                         return _ex0();
 420
 421                 // Expand product, if necessary
 422                 ex e_expanded = e.expand();
 423                 if (!is_ex_of_type(e_expanded, ncmul))
 424                         return dirac_trace(e_expanded, rl, trONE);
 425
 426                 // gamma5 gets moved to the front so this check is enough
 427                 bool has_gamma5 = is_ex_of_type(e.op(0).op(0), diracgamma5);
 428                 unsigned num = e.nops();
 429
 430                 if (has_gamma5) {
 431
 432                         // Trace of gamma5 * odd number of gammas and trace of
 433                         // gamma5 * gamma.mu * gamma.nu are zero
 434                         if ((num & 1) == 0 || num == 3)
 435                                 return _ex0();
 436
 437                         // Tr gamma5 gamma.mu gamma.nu gamma.rho gamma.sigma = 4I * epsilon(mu, nu, rho, sigma)
 438                         if (num == 5)
 439                                 return trONE * I * eps0123(e.op(1).op(1), e.op(2).op(1), e.op(3).op(1), e.op(4).op(1));
 440
 441                         // Tr gamma5 S_2k =
 442                         //   I/4! * epsilon0123.mu1.mu2.mu3.mu4 * Tr gamma.mu1 gamma.mu2 gamma.mu3 gamma.mu4 S_2k
 443                         exvector ix;
 444                         ix.reserve(num - 1);
 445                         for (unsigned i=1; i<num; i++)
 446                                 ix.push_back(e.op(i).op(1));
 447                         num--;
 448                         int *iv = new int[num];
 449                         ex result;
 450                         for (unsigned i=0; i<num-3; i++) {
 451                                 ex idx1 = ix[i];
 452                                 for (unsigned j=i+1; j<num-2; j++) {
 453                                         ex idx2 = ix[j];
 454                                         for (unsigned k=j+1; k<num-1; k++) {
 455                                                 ex idx3 = ix[k];
 456                                                 for (unsigned l=k+1; l<num; l++) {
 457                                                         ex idx4 = ix[l];
 458                                                         iv[0] = i; iv[1] = j; iv[2] = k; iv[3] = l;
 459                                                         exvector v;
 460                                                         v.reserve(num - 4);
 461                                                         for (unsigned n=0, t=4; n<num; n++) {
 462                                                                 if (n == i || n == j || n == k || n == l)
 463                                                                         continue;
 464                                                                 iv[t++] = n;
 465                                                                 v.push_back(ix[n]);
 466                                                         }
 467                                                         int sign = permutation_sign(iv, iv + num);
 468                                                         result += sign * eps0123(idx1, idx2, idx3, idx4)
 469                                                                 * trace_string(v.begin(), num - 4);
 470                                                 }
 471                                         }
 472                                 }
 473                         }
 474                         delete[] iv;
 475                         return trONE * I * result;
 476
 477                 } else { // no gamma5
 478
 479                         // Trace of odd number of gammas is zero
 480                         if ((num & 1) == 1)
 481                                 return _ex0();
 482
 483                         // Tr gamma.mu gamma.nu = 4 g.mu.nu
 484                         if (num == 2)
 485                                 return trONE * lorentz_g(e.op(0).op(1), e.op(1).op(1));
 486
 487                         exvector iv;
 488                         iv.reserve(num);
 489                         for (unsigned i=0; i<num; i++)
 490                                 iv.push_back(e.op(i).op(1));
 491
 492                         return trONE * trace_string(iv.begin(), num);
 493                 }
 494
 495         } else if (e.nops() > 0) {
 496
 497                 // Trace maps to all other container classes (this includes sums)
 498                 pointer_to_map_function_2args<unsigned char, const ex &> fcn(dirac_trace, rl, trONE);
 499                 return e.map(fcn);
 500
 501         } else
 502                 return _ex0();
 503 }
 504
 505 ex canonicalize_clifford(const ex & e)
 506 {
 507         // Scan for any ncmul objects
 508         lst srl;
 509         ex aux = e.to_rational(srl);
 510         for (unsigned i=0; i<srl.nops(); i++) {
 511
 512                 ex lhs = srl.op(i).lhs();
 513                 ex rhs = srl.op(i).rhs();
 514
 515                 if (is_ex_exactly_of_type(rhs, ncmul)
 516                  && rhs.return_type() == return_types::noncommutative
 517                  && is_clifford_tinfo(rhs.return_type_tinfo())) {
 518
 519                         // Expand product, if necessary
 520                         ex rhs_expanded = rhs.expand();
 521                         if (!is_ex_of_type(rhs_expanded, ncmul)) {
 522                                 srl.let_op(i) = (lhs == canonicalize_clifford(rhs_expanded));
 523                                 continue;
 524
 525                         } else if (!is_ex_of_type(rhs.op(0), clifford))
 526                                 continue;
 527
 528                         exvector v;
 529                         v.reserve(rhs.nops());
 530                         for (unsigned j=0; j<rhs.nops(); j++)
 531                                 v.push_back(rhs.op(j));
 532
 533                         // Stupid recursive bubble sort because we only want to swap adjacent gammas
 534                         exvector::iterator it = v.begin(), next_to_last = v.end() - 1;
 535                         if (is_ex_of_type(it->op(0), diracgamma5))
 536                                 ++it;
 537                         while (it != next_to_last) {
 538                                 if (it[0].op(1).compare(it[1].op(1)) > 0) {
 539                                         ex save0 = it[0], save1 = it[1];
 540                                         it[0] = lorentz_g(it[0].op(1), it[1].op(1));
 541                                         it[1] = _ex2();
 542                                         ex sum = ncmul(v);
 543                                         it[0] = save1;
 544                                         it[1] = save0;
 545                                         sum -= ncmul(v, true);
 546                                         srl.let_op(i) = (lhs == canonicalize_clifford(sum));
 547                                         goto next_sym;
 548                                 }
 549                                 ++it;
 550                         }
 551 next_sym:       ;
 552                 }
 553         }
 554         return aux.subs(srl);
 555 }
 556
 557 } // namespace GiNaC