A331815 Numbers k such that 10^(2*k) - 8*10^(k-1) - 1 is prime.

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.

Links

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

Index to OEIS entries related to primes involving repdigits.

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

Adjacent sequences: A331812 A331813 A331814 * A331816 A331817 A331818

Keyword

nonn,base,more

Author

Eder Vanzei, Jan 27 2020

Extensions

a(7) from Giovanni Resta, Jan 28 2020

Status

approved