A177415 Smallest prime divisor of the n-th base-2 pseudoprime.

11, 3, 3, 5, 19, 7, 3, 23, 5, 37, 7, 29, 37, 17, 3, 31, 43, 7, 73, 53, 3, 7, 31, 5, 5, 3, 7, 59, 11, 43, 23, 7, 5, 3, 97, 71, 3, 97, 3, 13, 7, 17, 89, 73, 103, 3, 5, 89, 5, 7, 5, 127, 13, 157, 151, 7, 3, 7, 101, 89, 3, 7, 59, 97, 11, 3, 5, 11, 61, 167, 5, 53, 3, 149, 11, 151

Offset: 1

Author

Juri-Stepan Gerasimov, Dec 10 2010

Keywords

• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A001567, A020639.

Programs

Mathematica

lpf[n_]:=FactorInteger[n][[1, 1]]; lpf /@ Select[Range[1, 10^4, 2], CompositeQ[#] && PowerMod[2, #-1, # ] == 1 &] (* Amiram Eldar, Sep 08 2019 *)

PARI

terms(n) = my(i=0); forcomposite(c=1, , if(i>=n, break, if(Mod(2, c)^(c-1)==1, print1(factor(c)[1, 1], ", "); i++)))
/* Print initial 80 terms as follows */
terms(80) \\ Felix Fröhlich, Sep 08 2019

Formula

a(n) = A020639(A001567(n)).

Extensions

Corrected by D. S. McNeil, Dec 11 2010