A028989 Smallest palindromic prime with 2n-1 digits.

2, 101, 10301, 1003001, 100030001, 10000500001, 1000008000001, 100000323000001, 10000000500000001, 1000000008000000001, 100000000212000000001, 10000000000300000000001, 1000000000016100000000001, 100000000000080000000000001, 10000000000000300000000000001

Offset: 1

Author

Patrick De Geest

Keywords

• nonn
• base

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..500
• Patrick De Geest, World!Of Palindromic Primes.

Crossrefs

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

Cf. A100027.

Programs

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 *)

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(xJeppe Stig Nielsen, Feb 20 2021

Extensions

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