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A213015 Numbers n such that the sum of prime factors of n (counted with multiplicity) is 2 times a prime. 3
4, 8, 9, 21, 25, 30, 32, 33, 36, 49, 57, 69, 70, 84, 85, 93, 100, 102, 120, 121, 128, 129, 133, 135, 144, 145, 162, 169, 174, 177, 182, 190, 205, 213, 217, 228, 237, 238, 246, 249, 253, 260, 265, 286, 289, 308, 309, 310, 312, 318, 340, 351, 361, 372, 393, 406 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The numbers A100118(n)^2 are in the sequence.

LINKS

Table of n, a(n) for n=1..56.

FORMULA

sopfr(n) = 2*p, p prime.

EXAMPLE

36 is in the sequence because 36 = 2^2 * 3^2 => sum of prime factors = 2*2+3*2 = 10 = 2*5 where 5 is prime.

MAPLE

with(numtheory):A:= proc(n) local e, j; e := ifactors(n)[2]: add (e[j][1]*e[j][2], j=1..nops(e)) end: for m from 1 to 3000 do: if type(A(m)/2, prime)= true then printf(`%d, `, m):else fi:od:

MATHEMATICA

L = {}; Do[ww = Transpose[FactorInteger[k]]; w = ww[[1]].ww[[2]]; If[PrimeQ[w/2], AppendTo[L, k]], {k, 2, 500}]; L

CROSSREFS

Cf. A001414, A100118.

Sequence in context: A089765 A116030 A116020 * A064393 A173743 A035326

Adjacent sequences:  A213012 A213013 A213014 * A213016 A213017 A213018

KEYWORD

nonn

AUTHOR

Michel Lagneau, Jun 02 2012

STATUS

approved

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Last modified October 14 09:57 EDT 2019. Contains 327995 sequences. (Running on oeis4.)