cp's OEIS Frontend

A342221 Integers k, not congruent to 1 mod 3, such that there is no prime of length k using digit 3 once and rest all 1.

%I A342221 #32 Mar 15 2021 12:21:36
%S A342221 15,26,32,41,68,93,99,113,116,119,134,150,158,161,170,173,176,177,179,
%T A342221 204,213,252,257,266,284,299,305,312,320,333,353,357,374,392,402,404,
%U A342221 419,434,443,450,491,495,506,509,513,518,527,548,551,554,570,582,584,593
%N A342221 Integers k, not congruent to 1 mod 3, such that there is no prime of length k using digit 3 once and rest all 1.
%C A342221 The "not congruent to 1 mod 3" condition is due to the fact that A090537(3*k+1) = 0.
%o A342221 (Python)
%o A342221 from sympy import isprime
%o A342221 A342221_list = []
%o A342221 for k in range(1,10**3):
%o A342221     if k % 3 != 1:
%o A342221         m, l = (10**k-1)//9, 2
%o A342221         for i in range(k):
%o A342221             if isprime(m+l):
%o A342221                 break
%o A342221             l *= 10
%o A342221         else:
%o A342221             A342221_list.append(k) # _Chai Wah Wu_, Mar 15 2021
%Y A342221 Cf. A090537, A107689.
%Y A342221 Subsequence of A007494.
%K A342221 nonn,base
%O A342221 1,1
%A A342221 _Michel Marcus_, Mar 14 2021
%E A342221 More terms from _Chai Wah Wu_, Mar 15 2021