

A007680


a(n) = (2n+1)*n!.
(Formerly M2861)


34



1, 3, 10, 42, 216, 1320, 9360, 75600, 685440, 6894720, 76204800, 918086400, 11975040000, 168129561600, 2528170444800, 40537905408000, 690452066304000, 12449059983360000, 236887827111936000, 4744158915944448000, 99748982335242240000
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OFFSET

0,2


COMMENTS

Denominators in series for sqrt(Pi/4)*erf(x): sqrt(Pi/4)*erf(x)= x/1  x^3/3 + x^5/10  x^7/42 + x^9/216 + ... This series is famous for its bad convergence if x > 1.
Appears to be the BinomialMean transform of A000354 (after truncating the first term of A000354). (See A075271 for the definition of BinomialMean.)  John W. Layman, Apr 16 2003
Number of permutations p of {1,2,...,n+2} such that maxp(i)i=n+1. Example: a(1)=3 since only the permutations 312,231 and 321 of {1,2,3} satisfy the given condition.  Emeric Deutsch, Jun 04 2003
Stirling transform of A000670(n+1) = [3, 13, 75, 541, ...] is a(n) = [3, 10, 42, 216, ...].  Michael Somos, Mar 04 2004
Stirling transform of a(n) = [2, 10, 42, 216, ...] is A052875(n+1) = [2, 12, 74, ...].  Michael Somos, Mar 04 2004
A related sequence also arises in evaluating indefinite integrals of sec(x)^(2k+1), k=0, 1, 2, ... Letting u = sec(x) and d = sqrt(u^21), one obtains a(0) = log(u+d) 2*k*a(k) = (2*k1)*u^(2*k1)*d + a(k1). Viewing these as polynomials in u gives 2^k*k!*a(k) = a(0) + d*Sum(i=0..k1){ (2*i+1)*i!*2^i*u^(2*i+1) }, which is easily proved by induction. Apart from the power of 2, which could be incorporated into the definition of u (or by looking at erf(ix/2)/ i (i=sqrt(1)), the sum's coefficients form our series and are the reciprocals of the power series terms for sqrt(Pi/4)*erf(ix/2)). This yields a direct but somewhat mysterious relationship between the power series of erf(x) and integrals involving sec(x).  William A. Huber (whuber(AT)quantdec.com), Mar 14 2002
When written in factoradic ("factorial base"), this sequence from a(1) onwards gives the smallest number containing two adjacent digits, increasing when read from left to right, whose difference is n1.  Christian Perfect, May 03 2016
a(n1)^2 is the number of permutations p of [1..2n] such that Sum_{i=1..2n} abs(p(i)i) = 2n^22.  Fang Lixing, Dec 07 2018
A standard series for the calculation of coordinates on a clothoid (also called cornuspiral):
x = s*(a(0)  (tau^2/a(2)) + (tau^4/a(4)  (tau^6/a(6)) + …)
y = s*((tau/a(1)) + (tau^3/a(3))  (tau^5/a(5)) + …).
s is the arclength from the clothoids origin to the desired point p(x,y). The tangent at the clothoids origin intersects with the tangent at the point p(x,y) with an angle of tau.  Thomas Scheuerle, Oct 13 2021
a(n) = P_n(1) where P_n(x) is the Pidduck polynomials.  Michael Somos, May 27 2023


REFERENCES

H. W. Gould, A class of binomial sums and a series transform, Utilitas Math., 45 (1994), 7183.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
N. Wirth, Systematisches Programmieren, 1975, exercise 9.3


LINKS

Emeric Deutsch, Problem Q915, Math. Magazine, vol. 74, No. 5, 2001, p. 404.
Eric Weisstein's World of Mathematics, Erf


FORMULA

E.g.f.: (1+x)/(1x)^2.
This is the binomial mean transform of A000354 (after truncating the first term). See Spivey and Steil (2006).  Michael Z. Spivey (mspivey(AT)ups.edu), Feb 26 2006
E.g.f.: (of aerated sequence) 1+x^2/2+sqrt(pi)*(x+x^3/4)*exp(x^2/4)*ERF(x/2).  Paul Barry, Apr 11 2010
G.f.: 1 + x*G(0), where G(k)= 1 + x*(k+1)/(1  (k+2)/(k+2 + (k+1)/G(k+1) )); (continued fraction).  Sergei N. Gladkovskii, Jul 08 2013
Dfinite with recurrence: (2*n+1)*a(n) +n*(2*n+1)*a(n1)=0.  R. J. Mathar, Jan 27 2020


EXAMPLE

G.f. = 1 + 3*x + 10*x^2 + 42*x^3 + 216*x^4 + 1320*x^5 + 9360*x^6 + ...  Michael Somos, Jan 01 2019


MAPLE



MATHEMATICA



PROG

(PARI) {a(n) = if( n<0, 0, (2*n+1) * n!)}; /* Michael Somos, Mar 04 2004 */
(GAP) a:=List([0..20], n>(2*n+1)*Factorial(n));; Print(a); # Muniru A Asiru, Jan 01 2019


CROSSREFS

(End)


KEYWORD

nonn,easy


AUTHOR



STATUS

approved



