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A117927 a(n) = binomial(s(n), n) where s(n) = n-th semiprime. 1
4, 15, 84, 210, 2002, 5005, 116280, 319770, 2042975, 5311735, 193536720, 548354040, 1476337800, 9669554100, 25140840660, 991493848554, 6499270398159, 27900908274925, 280576272201225, 1210269541711230, 3342649210440540 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Semiprime analog of A060604, binomial(p(n), n) where p(n) = n-th prime.

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

FORMULA

a(n) = binomial(A001358(n), n).

EXAMPLE

a(1) = binomial(4,1) = 4.

a(2) = binomial(6,2) = 15.

a(3) = binomial(9,3) = 84.

a(4) = binomial(10,4) = 210.

a(5) = binomial(14,5) = 2002.

a(6) = binomial(15,6) = 5005.

a(7) = binomial(21,7) = 116280.

a(8) = binomial(22,8) = 319770.

a(9) = binomial(25,9) = 2042975.

a(10) = binomial(26,10) = 5311735.

MATHEMATICA

Binomial[First[#], Last[#]]&/@With[{sprs=Select[Range[70], PrimeOmega[#] == 2&]}, Thread[{sprs, Range[Length[sprs]]}]] (* Harvey P. Dale, Aug 10 2012 *)

CROSSREFS

Cf. A001358, A060604.

Sequence in context: A232220 A129653 A081722 * A143340 A151379 A130679

Adjacent sequences:  A117924 A117925 A117926 * A117928 A117929 A117930

KEYWORD

easy,nonn

AUTHOR

Jonathan Vos Post, May 03 2006

STATUS

approved

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Last modified January 26 05:10 EST 2020. Contains 331273 sequences. (Running on oeis4.)