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A260852 Primes in A260851: numbers whose base n expansion is equal to the concatenation of the base n expansions of (1, 2, 3, ..., n-1, n, n-1, ..., 3, 2, 1). 11
13, 439, 27961, 522134761, 21107054541321649, 12345678910987654321, 24197857203266734883076090685781525281 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Primes of the form (1+r(b)*b)*(r(b)-b+1)-1 with r(b)=(b^b-1)/(b-1).

Sequences A173427, A260853 - A260859, A173426, A260861 - A260866, A260860 list the numbers whose base b expansion is the concatenation of the base b expansions of (1, 2, ..., n, n-1, ..., 1). For n < b these are the squares of the repdigits of length n in base b, so the first candidate is the b-th term. These are the numbers listed in A260851. For the bases listed in A260343, this candidate is indeed prime: these are the primes listed here.

a(8) = A260851(40) has already 127 digits and is therefore too large to be displayed here.

LINKS

Table of n, a(n) for n=1..7.

D. Broadhurst, Primes from concatenation: results and heuristics, NmbrThry List, August 1, 2015

FORMULA

a(n) = A260851(A260343(n)).

PROG

(PARI) for(b=2, 999, ispseudoprime(p=(1+b*c=(b^b-1)\(b-1))*(c-b+1)-1)&&print1(p", "))

CROSSREFS

Cf. A173427, A260853 - A260859, A173426, A260861 - A260866, A260860, A260851.

Sequence in context: A114759 A260871 A260851 * A012832 A102075 A218586

Adjacent sequences:  A260849 A260850 A260851 * A260853 A260854 A260855

KEYWORD

nonn,base

AUTHOR

M. F. Hasler, Aug 02 2015

STATUS

approved

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Last modified November 20 05:07 EST 2019. Contains 329323 sequences. (Running on oeis4.)