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A260802 Odd numbers x = 2n - 1 such that the concatenation of A019519(n) and A038395(n-1) is prime.

%I A260802 #34 May 22 2025 10:21:43
%S A260802 3,13,19,21,67
%N A260802 Odd numbers x = 2n - 1 such that the concatenation of A019519(n) and A038395(n-1) is prime.
%e A260802 a(1) = 3 since 13_1 is prime;
%e A260802 a(2) = 13 since 135791113_1197531 is prime;
%e A260802 a(3) = 19 since 135791113151719_1715131197531 is prime.
%o A260802 (Python)
%o A260802 import sympy
%o A260802 n=1
%o A260802 while n>0:
%o A260802     s=str(n)
%o A260802     for m in range(n-2,0,-2):
%o A260802         s=str(m)+s+str(m)
%o A260802     p=int(s)
%o A260802     if sympy.isprime(p)==True:
%o A260802         print(n)
%o A260802     n=n+2
%Y A260802 Cf. A019519, A038395.
%K A260802 nonn,more,base
%O A260802 1,1
%A A260802 _Abhiram R Devesh_, Jul 31 2015