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URL: https://oeis.org/A086125

⇱ A086125 - OEIS

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A086125
Values of k such that f(k) is a prime, where f(1) = 1, f(i) = prime(i)*f(i-1) + 2.
1
2, 4, 8, 15, 31, 128, 163, 12284
OFFSET
1,1
COMMENTS
No additional terms up to k = 1000. - Harvey P. Dale, Feb 02 2019
No additional terms up to k = 20000. - Michael S. Branicky, May 31 2025
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 *)
CROSSREFS
The primes are in A086124.
Sequence in context: A092603 A298533 A259805 * A227236 A061030 A036661
KEYWORD
more,nonn
AUTHOR
Labos Elemer, Jul 23 2003
EXTENSIONS
Edited by Robert G. Wilson v, Jul 25 2003
a(8) from Michael S. Branicky, May 29 2025
STATUS
approved