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A226839 E.g.f.: exp( Sum_{n>=1} x^(n*(n+1)/2) / n! ). 1
1, 1, 1, 4, 13, 31, 271, 1576, 6049, 55693, 573661, 3839716, 36369301, 432793219, 3670898323, 47260464616, 758854978561, 8126729609401, 106290146259289, 1742497711849828, 22974498485218621, 454423040764317031, 8508721270142443351, 120131676428508219784, 2346431431552540513633 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

COMMENTS

E.g.f. may be written as: exp( Sum_{n>=1} Product_{k=1..n} x^k/k ).

Sum_{n>=0} a(n)/n! = e^(e-1) = 5.574941524760880...

LINKS

Table of n, a(n) for n=0..24.

FORMULA

a(n) == 1 (mod 3) (conjecture - valid up to n=1024; if true for n>=0, why?).

EXAMPLE

E.g.f.: A(x) = 1 + x + x^2/2! + 4*x^3/3! + 13*x^4/4! + 31*x^5/5! + 271*x^6/6! +...

where

log(A(x)) = x + x^3/2! + x^6/3! + x^10/4! + x^15/5! + x^21/6! + x^28/7! +...

PROG

(PARI) {a(n)=n!*polcoeff(exp(sum(m=1, n, x^(m*(m+1)/2)/m!)+x*O(x^n)), n)}

for(n=0, 30, print1(a(n), ", "))

CROSSREFS

Cf. A226838.

Sequence in context: A027998 A026567 A218958 * A270976 A272510 A036487

Adjacent sequences: A226836 A226837 A226838 * A226840 A226841 A226842

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Jun 19 2013

STATUS

approved

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Last modified November 29 11:57 EST 2022. Contains 358424 sequences. (Running on oeis4.)