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A174956 0 unless n is the k-th semiprime when a(n) = k. 12
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,6

COMMENTS

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

LINKS

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

FORMULA

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

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 *)

PROG

(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

(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

CROSSREFS

Cf. A049084.

Sequence in context: A278094 A245487 A074734 * A124182 A188429 A188430

Adjacent sequences:  A174953 A174954 A174955 * A174957 A174958 A174959

KEYWORD

nonn

AUTHOR

Reinhard Zumkeller, Apr 03 2010

STATUS

approved

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Last modified August 9 12:53 EDT 2022. Contains 356026 sequences. (Running on oeis4.)