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A179400 Primes which are the fourth element of a generalized Wieferich sequence. 1
331, 359, 1549, 1777, 2011, 6211, 7481, 10369, 13477, 19069, 20431, 22567, 28289, 32933, 39041, 40597, 77713, 96979, 101489 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

A generalized Wieferich sequence is an increasing sequence of primes p[1],p[2],... such that a=p[n+1] is a Wieferich prime to base b=p[n], i.e., a^(b-1)=1 (mod b^2).

LINKS

Table of n, a(n) for n=1..19.

D. Broadhurst, Re: 1993/2011 puzzle [and Puzzle 7], in primenumbers@yahoogroups.com, Jan 2011.

EXAMPLE

The smallest generalized Wieferich sequence of length 4 is (3,11,71,331): 3^10=1 (mod 11^2), 11^70=1 (mod 71^2), 71^330=1 (mod 331^2). Therefore, a(1)=331.

This can actually be extended with 359 to such a sequence of length 5, since 331^358=1 (mod 359^2). Therefore, the next such sequence is (11,71,331,359) and a(2)=359.

Then comes a(3)=1549 from the sequence (307, 487, 1069, 1549). Note that this sequence "starts later" than (197, 653, 1381, 1777) which yields a(4)=1777.

PROG

(PARI) fp(p)={ my(a=precprime(p)); while(Mod(a, p^2)^(p-1)-1 & a=precprime(a-1), ); a }

forprime(p=1, default(primelimit), a=p; for(c=1, 3, (a=fp(a))|next(2)); print1(p", "))

CROSSREFS

Cf. A001220, A174422 and references therein.

Sequence in context: A256586 A319718 A319921 * A139657 A140000 A248535

Adjacent sequences:  A179397 A179398 A179399 * A179401 A179402 A179403

KEYWORD

nonn,hard,more

AUTHOR

M. F. Hasler, Jan 10 2011

STATUS

approved

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Last modified January 17 14:12 EST 2019. Contains 319225 sequences. (Running on oeis4.)