A174956 0 unless n is the k-th semiprime when a(n) = k.

0, 0, 0, 1, 0, 2, 0, 0, 3, 4, 0, 0, 0, 5, 6, 0, 0, 0, 0, 0, 7, 8, 0, 0, 9, 10, 0, 0, 0, 0, 0, 0, 11, 12, 13, 0, 0, 14, 15, 0, 0, 0, 0, 0, 0, 16, 0, 0, 17, 0, 18, 0, 0, 0, 19, 0, 20, 21, 0, 0, 0, 22, 0, 0, 23, 0, 0, 0, 24, 0, 0, 0, 0, 25, 0, 0, 26, 0, 0, 0, 0, 27, 0, 0, 28, 29, 30, 0, 0, 0, 31, 0, 32, 33

Offset: 1

Author

Reinhard Zumkeller, Apr 03 2010

Keywords

• nonn

Comments

a(A001358(n)) = n; a(A100959(n)) = 0.

Links

• R. Zumkeller, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A049084.

Programs

Haskell

import Data.List (unfoldr)
a174956 n = a174956_list !! (fromInteger n - 1)
a174956_list = unfoldr x (1, 1, a001358_list) where
   x (i, z, ps'@(p:ps)) | i == p = Just (z, (i + 1, z + 1, ps))
                        | i /= p = Just (0, (i + 1, z, ps'))
-- Reinhard Zumkeller, Oct 27 2012

Mathematica

nn = 100; With[{tbl = Table[If[PrimeOmega[n] == 2, 1, 0], {n, nn}]},
Table[If[tbl[[i]] == 0, 0, Total[Take[tbl, i]]], {i, nn}]] (* Harvey P. Dale, Oct 13 2012 *)

PARI

first(n)=my(v=List(),u=vector(n)); forprime(p=2,n\2, forprime(q=2,min(p,n\p), listput(v,p*q))); v=Set(v); for(i=1,#v, u[v[i]]=i); u \\ Charles R Greathouse IV, Sep 02 2015

Formula

a(n) = A064911(n)*A072000(n).