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A115331 E.g.f.: exp(x+5/2*x^2). 4
1, 1, 6, 16, 106, 426, 3076, 15856, 123516, 757756, 6315976, 44203776, 391582456, 3043809016, 28496668656, 241563299776, 2378813448976, 21703877431056, 223903020594016, 2177251989389056, 23448038945820576, 241173237884726176 (list; graph; refs; listen; history; text; internal format)
OFFSET
0,3
COMMENTS
Term-by-term square of sequence with e.g.f.: exp(x+m/2*x^2) is given by e.g.f.: exp(x/(1-m*x))/sqrt(1-m^2*x^2) for all m.
a(n) is also the number of square roots of any permutation in S_{5n} whose disjoint cycle decomposition consists of n cycles of length 5. - Luis Manuel Rivera Martínez, Feb 26 2015
LINKS
Jesús Leaños, Rutilo Moreno, and Luis Manuel Rivera-Martínez, On the number of mth roots of permutations, arXiv:1005.1531 [math.CO], 2010-2011.
Jesús Leaños, Rutilo Moreno, and Luis Manuel Rivera-Martínez, On the number of mth roots of permutations , Australas. J. Combin. 52 (2012), 41-54 (Theorems 1 and 2).
FORMULA
Term-by-term square equals A115332 which has e.g.f.: exp(x/(1-5*x))/sqrt(1-25*x^2).
a(n) ~ exp(sqrt(n/5)-n/2-1/20)*5^(n/2)*n^(n/2)/sqrt(2). - Vaclav Kotesovec, Oct 19 2012
a(n) = n!*Sum_{k=0..floor(n/2)}5^k/(2^k*k!*(n-2*k)!). - Luis Manuel Rivera Martínez, Feb 26 2015
O.g.f.: 1/(1-x - 5*x^2/(1-x - 10*x^2/(1-x - 15*x^2/(1-x - 20*x^2/(1-x - 25*x^2/(1-x -...)))))), a continued fraction (after Paul Barry in A115327). - Paul D. Hanna, Mar 08 2015
MATHEMATICA
Range[0, 20]! CoefficientList[Series[Exp[(x + 5 / 2 x^2)], {x, 0, 20}], x] (* Vincenzo Librandi, May 22 2013 *)
PROG
(PARI) a(n)=local(m=5); n!*polcoeff(exp(x+m/2*x^2+x*O(x^n)), n)
CROSSREFS
Column k=5 of A359762.
Cf. A115332.
Sequence in context: A009354 A034191 A368875 * A239027 A218976 A173737
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Jan 20 2006
STATUS
approved

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Last modified March 29 01:36 EDT 2024. Contains 371264 sequences. (Running on oeis4.)