A086125 Values of k such that f(k) is a prime, where f(1) = 1, f(i) = prime(i)*f(i-1) + 2.
2, 4, 8, 15, 31, 128, 163, 12284
Offset: 1
Programs
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Mathematica
f[1]=1; f[x_] := f[x] = Prime[x]*f[x - 1] + 2; Do[ If[ PrimeQ[ f[n]], Print[n]], {n, 1, 1900}] nxt[{n_,a_}]:={n+1,a*Prime[n+1]+2}; Select[NestList[nxt,{1,1},200], PrimeQ[ #[[2]]]&][[All,1]] (* Harvey P. Dale, Feb 02 2019 *)
Extensions
Edited by Robert G. Wilson v, Jul 25 2003
a(8) from Michael S. Branicky, May 29 2025
Comments