A342221 - OEIS

%I #32 Mar 15 2021 12:21:36

%S 15,26,32,41,68,93,99,113,116,119,134,150,158,161,170,173,176,177,179,

%T 204,213,252,257,266,284,299,305,312,320,333,353,357,374,392,402,404,

%U 419,434,443,450,491,495,506,509,513,518,527,548,551,554,570,582,584,593

%N 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 The "not congruent to 1 mod 3" condition is due to the fact that A090537(3*k+1) = 0.

%o (Python)

%o from sympy import isprime

%o A342221_list = []

%o for k in range(1,10**3):

%o     if k % 3 != 1:

%o         m, l = (10**k-1)//9, 2

%o         for i in range(k):

%o             if isprime(m+l):

%o                 break

%o             l *= 10

%o         else:

%o             A342221_list.append(k) # _Chai Wah Wu_, Mar 15 2021

%Y Cf. A090537, A107689.

%Y Subsequence of A007494.

%K nonn,base

%O 1,1

%A _Michel Marcus_, Mar 14 2021

%E More terms from _Chai Wah Wu_, Mar 15 2021