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A331815
Numbers k such that 10^(2*k) - 8*10^(k-1) - 1 is prime.
0
3, 4, 132, 471, 1935, 4258, 9444
OFFSET
1,1
COMMENTS
Also numbers k such that the concatenation (k 9's)1(k-1 9's) is prime.
EXAMPLE
3 is a term because 999199 is prime.
4 is a term because 99991999 is prime.
MATHEMATICA
Select[Range[500], PrimeQ[10^(2*#) - 8*10^(#-1) - 1] &] (* Amiram Eldar, Jan 28 2020 *)
PROG
(PARI) (is_A331815(n)=ispseudoprime(100^n-8*10^(n-1)-1)); for(n=1, 9999, is_A331815(n)&&print1(n", "))
CROSSREFS
Cf. A000040.
Cf. A077776 = A183184*2+1: palindromic near-repdigit primes 9..919..9.
Sequence in context: A280735 A356073 A290282 * A077032 A370385 A041595
KEYWORD
nonn,base,more
AUTHOR
Eder Vanzei, Jan 27 2020
EXTENSIONS
a(7) from Giovanni Resta, Jan 28 2020
STATUS
approved