A380011 Beginning with 7, least prime such that the reversal concatenation of the first n terms is prime.

7, 3, 3, 13, 3, 2, 13, 47, 43, 47, 37, 41, 109, 41, 139, 149, 109, 263, 73, 563, 163, 41, 19, 797, 61, 107, 31, 821, 43, 149, 37, 953, 211, 89, 547, 353, 337, 167, 67, 239, 1009, 449, 97, 23, 349, 41, 31, 911, 61, 929, 229, 797, 331, 191, 463, 107, 463, 809, 2887, 971

Offset

1,1

Comments

"Reverse concatenation" here refers to the decimal concatenation R(a(n)) || R(a(n-1)) || ... || R(a(3)) || R(a(2)) || R(a(1)) where R(k) means "reverse digits of k".

Links

J.W.L. (Jan) Eerland, Table of n, a(n) for n = 1..982

Mathematica

w={7}; Do[k=1; q=Monitor[Parallelize[While[True, If[PrimeQ[FromDigits[Join@@IntegerDigits/@Reverse[IntegerDigits[FromDigits[Join@@IntegerDigits/@Append[w, Prime[k]]]]]]], Break[]]; k++]; Prime[k]], k]; w=Append[w, q], {i, 2, 50}]; w

Prog

(Python)
from itertools import count, islice
from gmpy2 import digits, is_prime, mpz, next_prime
def agen(): # generator of terms
    r, an = "", 7
    while True:
        yield int(an)
        r = digits(an)[::-1] + r
        p = 2
        while not is_prime(mpz(digits(p)[::-1]+r)): p = next_prime(p)
        an = p
print(list(islice(agen(), 50))) # after Michael S. Branicky in A379355

Crossrefs

Cf. A111382, A111383, A113584, A379354, A379355, A379761, A380010.

Sequence in context: A153858 A200294 A169813 * A097517 A379761 A127559

Adjacent sequences: A380007 A380009 A380010 * A380012 A380013 A380014

Keyword

base,nonn

Author

J.W.L. (Jan) Eerland, Jan 09 2025

Status

approved