A331815 - OEIS

A331815

Numbers k such that 10^(2*k) - 8*10^(k-1) - 1 is prime.

3, 4, 132, 471, 1935, 4258, 9444

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OFFSET

1,1

COMMENTS

Also numbers k such that the concatenation (k 9's)1(k-1 9's) is prime.

LINKS

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. A077776 = A183184*2+1: palindromic near-repdigit primes 9..919..9.

KEYWORD

nonn,base,more

AUTHOR

Eder Vanzei, Jan 27 2020

EXTENSIONS

a(7) from Giovanni Resta, Jan 28 2020

STATUS

approved