%I #19 Feb 22 2020 22:11:22

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

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

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

%H <a href="/index/Pri#primes_involving_repunits_.2C_sequences_related_to_">Index to OEIS entries related to primes involving repdigits</a>.

%e 3 is a term because 999199 is prime.

%e 4 is a term because 99991999 is prime.

%t Select[Range[500], PrimeQ[10^(2*#) - 8*10^(#-1) - 1] &] (* _Amiram Eldar_, Jan 28 2020 *)

%o (PARI) (is_A331815(n)=ispseudoprime(100^n-8*10^(n-1)-1)); for(n=1, 9999, is_A331815(n)&&print1(n", "))

%Y Cf. A000040.

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

%K nonn,base,more

%O 1,1

%A _Eder Vanzei_, Jan 27 2020

%E a(7) from _Giovanni Resta_, Jan 28 2020