A002020 a(n+1) = a(n) - n*(n-1)*a(n-1), with a(n) = 1 for n <= 3. (Formerly M3876 N1588)

1, 1, 1, 1, -5, -17, 83, 593, -2893, -36101, 172195, 3421285, -15520165, -467129785, 1954015955, 86971636825, -323371713725, -21196564551725, 66760541581475, 6552909294409325, -16279195926455125, -2506384727801998625, 4330877561309153875

Offset

0,5

References

R. Kelisky, The numbers generated by exp(arctan x), Duke Math. J., 26 (1959), 569-581.

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Links

T. D. Noe, Table of n, a(n) for n = 0..100

Maple

f := proc(n) option remember; if n<=3 then 1 else f(n-1)-(n-1)*(n-2)*f(n-2); fi; end;

Mathematica

t = {1, 1, 1, 1}; Do[AppendTo[t, t[[-1]] - (n-1) (n - 2) t[[-2]]], {n, 4, 20}] (* T. D. Noe, Aug 30 2012 *)

Crossrefs

Sequence in context: A149755 A365878 A363163 * A038183 A149756 A036756

Adjacent sequences: A002017 A002018 A002019 * A002021 A002022 A002023

Keyword

sign,easy

Author

N. J. A. Sloane

Status

approved