OFFSET
1,1
COMMENTS
Semiprime analog of A060604.
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 *)
PROG
(PARI) s(n) = my(k=1, nb=0); while (nb != n, if (bigomega(k)==2, nb++); if (nb != n, k++)); k; \\ A001358
a(n) = binomial(s(n), n); \\ Michel Marcus, Jul 15 2022
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, May 03 2006
EXTENSIONS
More terms from Michel Marcus, Jul 15 2022
STATUS
approved