A028990 Largest palindromic prime with 2n-1 digits.

7, 929, 98689, 9989899, 999727999, 99999199999, 9999987899999, 999999787999999, 99999999299999999, 9999999992999999999, 999999999757999999999, 99999999997579999999999

Offset

1,1

Comments

For n>1, since 11 is a proper divisor of a palindrome with 2n digits, the numbers a(n) and A028989(n+1) are consecutive in A002385. - Jeppe Stig Nielsen, Feb 20 2021

Links

Jeppe Stig Nielsen, Table of n, a(n) for n = 1..500

Patrick De Geest, World!Of Palindromic Primes

Prog

(PARI) a(n)=forstep(j=10^n-1, 10^(n-1), -1, my(d=digits(j), p=fromdigits(vector(2*n-1, x, if(x<n, d[x], d[2*n-x])))); ispseudoprime(p)&&return(p)); 0 \\ Jeppe Stig Nielsen, Feb 20 2021

Crossrefs

Cf. A002385, A028989.

Sequence in context: A252175 A038808 A068728 * A304396 A151579 A024098

Adjacent sequences: A028987 A028988 A028989 * A028991 A028992 A028993

Keyword

nonn,base

Author

Patrick De Geest

Extensions

Offset changed (as it was in A028989) by Jeppe Stig Nielsen, Feb 20 2021

Status

approved