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A271385 a(n) = Product_{k=0..floor((n - 1)/2)} (n - 2*k)^(n - 2*k). 1
1, 1, 4, 27, 1024, 84375, 47775744, 69486440625, 801543976648704, 26920470805806965625, 8015439766487040000000000, 7680724499239438722449399746875, 71466466094944065310414602240000000000, 2326300251412874049290421829657963142035959375 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Double hyperfactorial (by analogy with the double factorial).

LINKS

Ilya Gutkovskiy, Table of n, a(n) for n = 0..33

Ilya Gutkovskiy, Double hyperfactorial

Eric Weisstein, Double Factorial

Eric Weisstein's World of Mathematics, Hyperfactorial

Index entries for sequences related to factorial numbers

FORMULA

a(n) = n^n*(n - 2)^(n - 2)*...*5^5*3^3*1^1, for n>0 odd; a(n) = n^n*(n - 2)^(n - 2)*...*6^6*4^4*2^2, for n>0 even; a(n) = 1, for n = 0.

a(n) = n^n*a(n-2), a(0)=1, a(1)=1.

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

a(n)*a(n-1) = A002109(n).

a(n)*a(n-1)*sqrt(a(2n))/((n!)^n*sqrt(2^(n*(n+1)))) = A168510(n).

EXAMPLE

a(0) = 1;

a(1) = 1^1 = 1;

a(2) = 2^2 = 4;

a(3) = 1^1*3^3 = 27;

a(4) = 2^2*4^4 = 1024;

a(5) = 1^1*3^3*5^5 = 84375;

a(6) = 2^2*4^4*6^6 = 47775744;

a(7) = 1^1*3^3*5^5*7^7 = 69486440625;

a(8) = 2^2*4^4*6^6*8^8 = 801543976648704, etc.

MATHEMATICA

Table[Product[(n - 2 k)^(n - 2 k), {k, 0, Floor[(n - 1)/2]}], {n, 0, 13}]

RecurrenceTable[{a[0] == 1, a[1] == 1, a[n] == n^n a[n - 2]}, a, {n, 13}]

PROG

(PARI) a(n) = prod(k=0, (n-1)\2, (n-2*k)^(n-2*k)); \\ Michel Marcus, Apr 07 2016

CROSSREFS

Cf. A002109, A006882, A168510.

Sequence in context: A068327 A066842 A133032 * A110763 A066352 A249105

Adjacent sequences:  A271382 A271383 A271384 * A271386 A271387 A271388

KEYWORD

nonn,easy

AUTHOR

Ilya Gutkovskiy, Apr 06 2016

STATUS

approved

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Last modified October 15 13:01 EDT 2018. Contains 316236 sequences. (Running on oeis4.)