A028989 Smallest palindromic prime with 2n-1 digits.

2, 101, 10301, 1003001, 100030001, 10000500001, 1000008000001, 100000323000001, 10000000500000001, 1000000008000000001, 100000000212000000001, 10000000000300000000001

Offset

1,1

Links

Charles R Greathouse IV, Table of n, a(n) for n = 1..500

Patrick De Geest, World!Of Palindromic Primes

Mathematica

t={}; Do[p=NextPrime[10^(2*n)]; While[Reverse[x=IntegerDigits[p]]!=x, p=NextPrime[p]]; AppendTo[t, p], {n, 0, 6}]; t (* Jayanta Basu, Jun 05 2013 *)

Prog

(PARI) a(n)={
n--;
my(N=100^n+1, aS=10^(n+3)+10^(n-3), bS=10^(n+2)+10^(n-2), cS=10^(n+1)+10^(n-1), dS=10^n);
forstep(a=N, N+9*aS, aS,
forstep(b=a, a+9*bS, bS,
forstep(c=b, b+9*cS, cS,
forstep(d=c, c+9*dS, dS,
if(ispseudoprime(d), return(d))
))));
warning("could not find a("n")")
}; \\ Charles R Greathouse IV, Feb 15 2011
(PARI) a(n)=for(j=10^(n-1), 10^n-1, 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

Odd-numbered terms of A056732. - Edward Catmur, May 01 2015

Sequence in context: A222449 A035325 A266457 * A113575 A173640 A111012

Adjacent sequences: A028986 A028987 A028988 * A028990 A028991 A028992

Keyword

nonn,base

Author

Patrick De Geest

Extensions

a(10) corrected by Farideh Firoozbakht, Oct 10 2005

Status

approved