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A220092
a(n) = ((2*n-1)!! + (-1)^((n-1)*(n-2)/2))/2.
1
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).
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
KEYWORD
nonn
AUTHOR
Michel Marcus, Dec 11 2012
STATUS
approved