OFFSET
0,2
COMMENTS
At the Mar 31 2011 suggestion of Zak Seidov in A086971.
Often a(n+1) = k*a(n) for some integer k.
All terms are cubefree products of primorials (A025487 INTERSECT A004709). - Charles R Greathouse IV, Dec 11 2012
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 0..10000
MATHEMATICA
semiPrimeQ[n_] := PrimeOmega@ n == 2; f[n_] := Length@ Select[Divisors@ n, semiPrimeQ@# &]; t = Table[0, {50}]; k = 1; While[k < 10^7, a = f@ k; If[t[[a]] == 0, t[[a]] = k; Print[{a, k}]]; k++]; t
PROG
(PARI) prim(n)=my(v=primes(n)); prod(i=1, #v, v[i])
a(n)=if(n>1, my(L=(sqrtint(8*n+1)+1)\2); prim(L)*prim(n-binomial(L, 2)), 1+3*n) \\ Charles R Greathouse IV, Dec 11 2012
(Haskell)
import Data.List (find); import Data.Maybe (fromJust)
a220264 n = fromJust $ find ((== n) . a086971) a220423_list
-- Reinhard Zumkeller, Sep 08 2015
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Robert G. Wilson v, Dec 09 2012
EXTENSIONS
a(25)-a(26) from Donovan Johnson, Dec 10 2012
a(27)-a(41) from Charles R Greathouse IV, Dec 11 2012
STATUS
approved