A220092 a(n) = ((2*n-1)!! + (-1)^((n-1)*(n-2)/2))/2.

1, 2, 7, 52, 473, 5198, 67567, 1013512, 17229713, 327364538, 6874655287, 158117071612, 3952926790313, 106729023338438, 3095141676814687, 95949391981255312, 3166329935381425313, 110821547738349885938, 4100397266318945779687, 159915493386438885407812

Offset

1,2

Comments

Described by V. Shevelev as an instance of a fast-growing nontrivial sequence that is similar to the naturals in the sense that A007814(a(n)) = A007814(n).

Links

Table of n, a(n) for n=1..20.

V. Shevelev, Several results on sequences which are similar to the positive integers, arXiv:0904.2101 [math.NT], 2009.

Prog

(Maxima) A220092(n):=((2*n-1)!!+(-1)^((n-1)*(n-2)/2))/2$ makelist(A220092(n), n, 1, 15); /* Martin Ettl, Dec 13 2012 */

Crossrefs

Cf. A007814.

Sequence in context: A224879 A345678 A279198 * A138737 A216086 A210856

Adjacent sequences: A220089 A220090 A220091 * A220093 A220094 A220095

Keyword

nonn

Author

Michel Marcus, Dec 11 2012

Status

approved