A271438 Numbers n such that the number formed by concatenating in ascending order the first n multidigit palindromic primes is prime.

1, 17, 451

Offset

1,2

Links

Table of n, a(n) for n=1..3.

G. L. Honaker, Jr. and C. Caldwell, 11101...10301 (52-digits), Prime Curios!.

Example

Concatenating in ascending order the 17 smallest palindromic primes with at least two digits gives 1110113115118119131335337338372775778779791992910301, which is prime, so 17 is a term of the sequence.

Prog

(PARI) nextpalprime(n) = my(p=nextprime(n)); while(p!=eval(concat(Vecrev(Str(p)))), p=nextprime(p+1)); p
terms(n) = my(i=0, k=1, s="", p=10); while(i < n, p=nextpalprime(p+1); s=Str(s, p); if(ispseudoprime(eval(s)), print1(k, ", "); i++); k++)
terms(3) \\ print initial three terms
(Python)
from sympy import isprime, nextprime
def ispal(n): strn = str(n); return strn == strn[::-1]
def agen():
  np, p, pstr = 1, 11, "11"
  while True:
    if isprime(int(pstr)): yield np
    p = nextprime(p)
    while not ispal(p): p = nextprime(p)
    np += 1
    pstr += str(p)
for an in agen(): print(an, end=", ") # Michael S. Branicky, Jan 17 2021

Crossrefs

Cf. A002385.

Sequence in context: A301642 A035022 A012117 * A122430 A009051 A035276

Adjacent sequences: A271435 A271436 A271437 * A271439 A271440 A271441

Keyword

nonn,base,more,bref

Author

Felix Fröhlich, Apr 07 2016

Status

approved