OFFSET
1,1
COMMENTS
a(26) <= 354208192001. - Donovan Johnson, Jun 03 2011
EXAMPLE
a(1) = 7 because 7 is prime and d(6) = 4 = d(8).
MATHEMATICA
s = Select[Prime@ Range@ 1000000, DivisorSigma[0, # - 1] == DivisorSigma[0, # + 1] &]; t = DivisorSigma[0, # - 1] & /@ s; a = {0}; b = {0}; Do[If[t[[k]] > Max@ b, AppendTo[a, s[[k]]]]; AppendTo[b, t[[k]]], {k, Length@ s}]; a (* Michael De Vlieger, Oct 30 2015 *)
PROG
(PARI) r=0; forprime(p=2, 4e9, t=numdiv(p-1); if(t>r&t==numdiv(p+1), r=t; print1(p", "))) \\ Charles R Greathouse IV, May 27 2011
CROSSREFS
KEYWORD
nonn
AUTHOR
Juri-Stepan Gerasimov, May 21 2011
EXTENSIONS
a(14)-a(21) from Charles R Greathouse IV, May 27 2011
a(22) from Charles R Greathouse IV, May 31 2011
a(23)-a(25) from Donovan Johnson, Jun 03 2011
STATUS
approved