The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A190680 Primes p such that sopfr(p-1) = sopfr(p+1) is also prime, where sopfr is A001414. 4
 11, 251, 1429, 906949, 1050449, 1058389, 3728113, 9665329, 13623667, 14320489, 30668003, 30910391, 45717377, 49437001, 55544959, 57510911, 58206653, 58772257, 69490901, 72191321, 73625789, 75235973, 79396433, 99673891, 103821169, 104662139, 121322449, 125938889, 147210257, 164810311, 169844879, 170650169, 201991721 (list; graph; refs; listen; history; text; internal format)
 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 Subsequence of A086711. Cf. A190722. Sequence in context: A098672 A056210 A182350 * A089298 A274129 A012154 Adjacent sequences:  A190677 A190678 A190679 * A190681 A190682 A190683 KEYWORD nonn AUTHOR Charles R Greathouse IV, May 16 2011 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified April 15 07:40 EDT 2021. Contains 342975 sequences. (Running on oeis4.)