

A213741


Numbers n such that the sum of the first n primes is divisible by exactly 3 prime powers (not including 1).


1



5, 13, 20, 23, 24, 35, 39, 41, 42, 43, 47, 50, 56, 61, 62, 63, 67, 68, 69, 70, 73, 76, 78, 81, 86, 90, 98, 112, 123, 126, 128, 134, 143, 145, 147, 160, 165, 166, 172, 176, 180, 182, 186, 189, 191, 193, 196, 197, 200, 215, 220, 222, 223, 225, 227, 229, 238
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OFFSET

1,1


COMMENTS



LINKS



FORMULA



EXAMPLE

a(1) = 5 because the sum of first 5 primes is 28 = 2^2 * 7 which has exactly three prime power factors (not including 1).
a(2) = 13 because the sum of first 13 primes is 238 = 2 * 7 * 17 which has exactly three prime power factors (not including 1).
a(3) = 20 because the sum of first 20 primes is 639 = 3^2 * 71.


MATHEMATICA

ps = 0; t = {}; Do[ps = ps + Prime[n]; If[Total[Transpose[FactorInteger[ps]][[2]]] == 3, AppendTo[t, n]], {n, 300}]; t (* T. D. Noe, Jun 27 2012 *)


PROG

(PARI) list(lim)=my(v=List(), k, s); forprime(p=2, prime(lim\1), k++; if(bigomega(s+=p)==3, listput(v, k))); Vec(v) \\ Charles R Greathouse IV, Feb 05 2017


CROSSREFS



KEYWORD

nonn,easy


AUTHOR



STATUS

approved



