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 A005413 Number of non-vanishing Feynman diagrams of order 2n+1 for the electron-electron-photon proper vertex function in quantum electrodynamics (QED). (Formerly M4445) 6
 1, 1, 7, 72, 891, 12672, 202770, 3602880, 70425747, 1503484416, 34845294582, 872193147840, 23469399408510, 676090493459712, 20771911997290116, 678287622406488192, 23466105907996232835, 857623856612704266240 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS From Robert Coquereaux, Sep 12 2014: (Start) QED diagrams are graphs with two kinds of edges (lines): a (non-oriented), f (oriented), and only one kind of (internal) vertex: aff. They may have internal and external (ie pendant) lines. QED diagrams containing loops of type f with an odd number of vertices are set to 0 (vanishing diagrams). Proper diagrams also called one-particle-irreducible diagrams (1PI) are connected diagrams that remain connected when an arbitrary internal line is cut. The proper vertex function of QED is described by proper (1PI) diagrams with one external line of type a (photon) and two external lines of type f (electron). Non-vanishing diagrams only exist if the number of vertices is odd. The number of non-vanishing Feynman diagrams for the proper vertex function is obtained from  g*Gamma(g) = g (1 + 1 g^2 + 7 g^4 + 72 g^6 + ...) where the exponent p of g^p gives the number of (internal) vertices, p is called the order of the diagram. Write g*Gamma(g) = g (1 + x + 7 x2 + 72 x3 + ...) with x = g^2. The sequence a(n) gives the coefficient of x^n. Relation with A005411: Gamma (g) =  (S(g) -1)/(g^2 S(g)^3) where S(g) = 1 + g^2 + 4 g^4+ 25 g^6+ ... is sum A005411(n) g^(2n), hence the G.f. in terms of modified Bessel functions. (End) REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). C. Itzykson and J.-B. Zuber, Quantum Field Theory, McGraw-Hill, 1980, pages 466-467. LINKS Robert Coquereaux, Table of n, a(n) for n = 0..250 P. Cvitanovic, B. Lautrup and R. B. Pearson, The number and weights of Feynman diagrams, Phys. Rev. D18, pp. 1939-1949 (1978). Kevin Hartnett, Physicists uncover strange numbers in particle collisions, Quanta Magazine, November 15 2016. R. J. Martin and M. J. Kearney, An exactly solvable self-convolutive recurrence, Aequat. Math., 80 (2010), 291-318. see p. 310. R. J. Martin and M. J. Kearney, An exactly solvable self-convolutive recurrence, arXiv:1103.4936 [math.CO], 2011. Wikipedia, Feynman diagram FORMULA See recurrence in Martin-Kearney paper. a(n) = (n - 1) * (A005412(n) + 2 * n * A005412(n - 1)) if n>1. From Robert Coquereaux, Sep 12 2014: (Start) The G.f. for this sequence is  (U - 1)/(U^3 x) where U is G.f. for A005411. G.f.: (4*x*(-2*x + (1 - K(1, -(1/(4*x))) / K(0, -(1/(4*x))))))/    (1 - K(1, -(1/(4*x))) / K(0, -(1/(4*x))))^3 where K(p, z) denotes the modified Bessel function of the second kind (order p, argument z). This is a small improvement of a result obtained in the 1980 book "Quantum Field Theory". (End) EXAMPLE G.f. = 1 + x + 7*x^2 + 72*x^3 + 891*x^4 + 12672*x^5 + 202770*x^6 + 3602880*x^7 + ... MATHEMATICA a[n_]:= SeriesCoefficient[(4*x*(-2*x + (1 - BesselK[1, -(1/(4*x))]/BesselK[0, -(1/(4*x))])))/ (1 - BesselK[1, -(1/(4*x))]/BesselK[0, -(1/(4*x))])^3, {x, 0, n}] (* Robert Coquereaux, Sep 12 2014 *) PROG (PARI) {a(n) = my(A); if( n<2, n>=0, A = vector(n); A = 1; for( k=2, n, A[k] = (2 * k - 2) * A[k-1] + sum( j=1, k-1, A[j] * A[k-j])); (n-1) * (A[n] + 2 * n * A[n-1]))}; /* Michael Somos, Jul 24 2011 */ (Haskell) a005413 n = a005413_list !! (n-1) a005413_list = 1 : zipWith (*) [1 ..]                            (zipWith (+) (tail a005412_list)                            (zipWith (*) [4, 6 ..] a005413_list)) -- Reinhard Zumkeller, Jan 24 2014 CROSSREFS Cvitanovic et al. relate this sequence to A000698 and A005411. - Robert Munafo, Jan 24 2010 Sequence in context: A242827 A077332 A222833 * A306377 A230890 A203473 Adjacent sequences:  A005410 A005411 A005412 * A005414 A005415 A005416 KEYWORD nonn,easy AUTHOR EXTENSIONS Name clarified and reference added by Robert Coquereaux, Sep 12 2014 STATUS approved

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Last modified November 18 04:43 EST 2019. Contains 329248 sequences. (Running on oeis4.)