OFFSET

1,1

COMMENTS

The first three terms were computed by J. M. Bergot (personal communication from J. M. Bergot to N. J. A. Sloane, May 16 2011).

The number of terms < 10^n: 0, 1, 2, 3, 3, 4, 8, 24, 70, 253, 839, ..., . - Robert G. Wilson v, May 31 2011

LINKS

Charles R Greathouse IV and Robert G. Wilson v, Table of n, a(n) for n = 1.. 2592 (Charles R Greathouse IV to 1536, Robert G. Wilson v to 2592)

EXAMPLE

sopfr(250) = sopfr(2*5^3) = 2 + 5*3 = 17 = 2*2 + 3*2 + 7 = sopfr(2^2*3^2*7) = sopfr(252), and 17 and 251 are prime, so 251 is in this sequence.

MATHEMATICA

f[n_] := Plus @@ Flatten[ Table[ #[[1]], {#[[2]]}] & /@ FactorInteger@ n]; fQ[n_] := Block[{pn = f[n - 1], pp = f[n + 1]}, pn == pp && PrimeQ@ pn]; p = 2; lst = {}; While[p < 216000000, If[ fQ@ p, AppendTo[lst, p]]; p = NextPrime@ p]; lst (* Robert G. Wilson v, May 18 2011 *)

CROSSREFS

KEYWORD

nonn

AUTHOR

Charles R Greathouse IV, May 16 2011

STATUS

approved