A254139 a(n) = smallest composite c for which there exist exactly n bases b with b < c such that b^(c-1) == 1 (mod c), i.e., smallest composite c which is a Fermat pseudoprime to exactly n bases less than c.

9, 28, 15, 66, 49, 232, 45, 190, 121, 276, 169, 1106

Offset

1,1

Comments

a(13) > 150000.

Links

Table of n, a(n) for n=1..12.

Example

With c = 49: there are exactly five bases b with b < 49 such that 49 is a Fermat pseudoprime, namely 18, 19, 30, 31 and 48. Since 49 is the smallest composite having exactly five such bases, a(5) = 49.

Prog

(PARI) for(n=1, 20, forcomposite(c=3, , b=2; i=0; while(b < c, if(Mod(b, c)^(c-1)==1, i++); b++); if(i==n, print1(c, ", "); break({1}))))

Crossrefs

Cf. A001567, A141768, A181780.

Sequence in context: A369721 A369759 A033479 * A034116 A031454 A044999

Adjacent sequences: A254136 A254137 A254138 * A254140 A254141 A254142

Keyword

nonn,more

Author

Felix Fröhlich, Jan 26 2015

Status

approved