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A260786 Twice the Euler or up/down numbers A000111. 4
2, 2, 2, 4, 10, 32, 122, 544, 2770, 15872, 101042, 707584, 5405530, 44736512, 398721962, 3807514624, 38783024290, 419730685952, 4809759350882, 58177770225664, 740742376475050, 9902996106248192, 138697748786275802, 2030847773013704704, 31029068327114173810, 493842960380415967232 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

LINKS

Table of n, a(n) for n=0..25.

S. T. Thompson, Problem E754: Skew Ordered Sequences, Amer. Math. Monthly, 54 (1947), 416-417. [Annotated scanned copy]

FORMULA

a(0)=a(1)=2; thereafter a(n) = (1/4)*Sum{k=1..n} binomial(n-1, k-1)*a(k-1)*a(n-k).

MAPLE

f:=proc(n) option remember;

if n <= 1 then 2 else (1/4)*add(binomial(n-1, k-1)*f(k-1)*f(n-k), k=1..n); fi;

end;

[seq(f(n), n=0..30)];

CROSSREFS

Cf. A000111.

Apart from initial terms, same as A001250.

Sequence in context: A307522 A130707 A131562 * A107902 A142974 A259856

Adjacent sequences:  A260783 A260784 A260785 * A260787 A260788 A260789

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Aug 04 2015

STATUS

approved

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Last modified August 13 04:54 EDT 2020. Contains 336442 sequences. (Running on oeis4.)