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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. 0
9, 28, 15, 66, 49, 232, 45, 190, 121, 276, 169, 1106 (list; graph; refs; listen; history; text; internal format)
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: A225300 A053825 A033479 * A034116 A031454 A044999

Adjacent sequences:  A254136 A254137 A254138 * A254140 A254141 A254142

KEYWORD

nonn,more

AUTHOR

Felix Fröhlich, Jan 26 2015

STATUS

approved

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Last modified June 16 19:43 EDT 2019. Contains 324155 sequences. (Running on oeis4.)