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 A092552 Let X_{m,n}(q) be the chromatic polynomial of the complete bipartite graph K_{m,n}. Then a(n) is the negative of the coefficient of the linear term of X_{n,n}(q). 12
 0, 1, 3, 31, 675, 25231, 1441923, 116914351, 12764590275, 1805409270031, 321113303226243, 70146437009397871, 18462286083671614275, 5762225835975165678031, 2104263061425865873128963, 888881838896989670838028591, 430058409024841744606172532675 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS Alois P. Heinz, Table of n, a(n) for n = 0..238 FORMULA From Alois P. Heinz, Apr 30 2012: (Start) a(n) = (-1) * [q] Sum_{j=1..n} (q-j)^n*S2(n,j)*Product_{i=0..j-1} (q-i). a(n) = (-1) * A212084(n,2n-1). (End) E.g.f.: Sum_{n>=1} (1 - exp(-n*x))^n / n. - Paul D. Hanna, Dec 06 2012 a(n) = Sum_{k=1..n} k!*(k-1)! * Stirling2(n, k)^2. - Paul D. Hanna, Dec 30 2012, corrected by Vaclav Kotesovec, Jun 21 2013 O.g.f.: Sum_{n>=1} n^(n-1) * n! * x^n / Product_{k=1..n} (1 + n*k*x). - Paul D. Hanna, Jan 05 2013 a(n) = A136126(2*n-1,n), where triangle A136126(n,k) is the number of permutations of {1,2,...,k+n} having excedance set {1,2,...,k}. - Paul D. Hanna, Feb 01 2013 a(n) ~ sqrt(Pi) * n^(2*n-1/2) / (sqrt(1-log(2)) * exp(2*n) * (log(2))^(2*n)). - Vaclav Kotesovec, Nov 07 2014 a(n) = A306209(2n-1,n-1) for n > 0. - Alois P. Heinz, Feb 01 2019 EXAMPLE a(2) = 3 since the chromatic polynomial of K_{2,2}(q) is q^4-4*q^3+6*q^2-3*q. E.g.f.: A(x) = x + 3*x^2/2! + 31*x^3/3! + 675*x^4/4! + 25231*x^5/5! +... where A(x) = (1-exp(-x)) + (1-exp(-2*x))^2/2 + (1-exp(-3*x))^3/3 +... - Paul D. Hanna, Dec 06 2012 O.g.f.: F(x) = x + 3*x^2 + 31*x^3 + 675*x^4 + 25231*x^5 +... where F(x) = x/(1+x) + 2^1*2!*x^2/((1+2*1*x)*(1+2*2*x)) + 3^2*3!*x^3/((1+3*1*x)*(1+3*2*x)*(1+3*3*x)) + 4^3*4!*x^4/((1+4*1*x)*(1+4*2*x)*(1+4*3*x)*(1+4*4*x)) +... - Paul D. Hanna, Jan 05 2013 MAPLE a:= n-> -coeff(add(Stirling2(n, k) *mul(q-i, i=0..k-1)              *(q-k)^n, k=1..n), q, 1): seq(a(n), n=0..20);  # Alois P. Heinz, Apr 30 2012 MATHEMATICA Table[Sum[k!*(k-1)!*StirlingS2[n, k]^2, {k, 1, n}], {n, 0, 20}] (* Vaclav Kotesovec, Jun 21 2013 *) PROG (PARI) {a(n)=n!*polcoeff(sum(k=1, n, (1-exp(-k*x+x*O(x^n)))^k/k), n)} \\ Paul D. Hanna, Dec 06 2012 for(n=0, 20, print1(a(n), ", ")) (PARI) {Stirling2(n, k)=n!*polcoeff(((exp(x+x*O(x^n))-1)^k)/k!, n)} {a(n)=if(n<=0, 0, sum(k=1, n, k!*(k-1)! * Stirling2(n, k)^2))} \\ Paul D. Hanna, Dec 30 2012 for(n=0, 20, print1(a(n), ", ")) (PARI) {a(n)=if(n<1, 0, polcoeff(sum(m=1, n, m^(m-1)*m!*x^m/prod(k=1, m, 1+m*k*x+x*O(x^n))), n))} \\ Paul D. Hanna, Jan 05 2013 CROSSREFS Cf. A008277, A212084, A220179, A136126, A306209. Sequence in context: A222895 A105293 A104841 * A322487 A300735 A196457 Adjacent sequences:  A092549 A092550 A092551 * A092553 A092554 A092555 KEYWORD nonn AUTHOR Michael Lugo (mtlugo(AT)mit.edu), Apr 09 2004 STATUS approved

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Last modified December 16 01:43 EST 2019. Contains 330013 sequences. (Running on oeis4.)