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
D. Broadhurst, Primes from concatenation: results and heuristics, NmbrThry List, August 1, 2015
PROG
(PARI) for(b=2, 999, ispseudoprime(p=(1+b*c=(b^b-1)\(b-1))*(c-b+1)-1)&&print1(p", "))
CROSSREFS
KEYWORD
nonn,base
AUTHOR
M. F. Hasler, Aug 02 2015
STATUS
approved