A177930 a(n) is the smallest prime divisor of (A177929(n)-1)*(A177929(n)+1).

3, 5, 3, 11, 3, 5, 3, 29, 3, 59, 3, 7, 5, 3, 7, 137, 3, 5, 3, 281, 3, 5, 3, 569, 3, 7, 5, 3, 1151, 3, 5, 3, 2309, 3, 31, 4649, 3, 17, 3, 7, 5, 3, 7, 5, 3, 9341, 3, 5, 3, 11, 3, 59, 3, 29, 3, 19, 7, 5, 3, 7, 19, 5, 3, 7, 109, 5, 3, 61, 7, 79, 5, 3, 13, 5, 3, 19139, 3, 101, 3, 7, 13, 11, 3, 71, 3

Offset: 1

Author

Vladimir Shevelev, May 15 2010

Keywords

• nonn

Comments

Records are in A177932, their positions in A177931.

Crossrefs

Cf. A001359, A020639, A177929, A174514, A174216, A174217

Programs

Mathematica

lpf[n_] := FactorInteger[n][[1, 1]];
b[n_] := b[n] = If[n == 1, 4, b[n-1]+lpf[b[n-1]^2-1]-1];
a[n_] := lpf[b[n]^2-1];
Table[a[n], {n, 1, 85}] (* Jean-François Alcover, May 13 2023 *)

Formula

a(n) = A020639( (A177929(n))^2 -1 ).

Extensions

A 7 replaced by 17 and sequence extended by R. J. Mathar, May 31 2010