A350592 Integers m such that b(m) := 20^m*(5^(m+1) - 1)/4 + (20^m - 1)/19 is a prime.
2, 4, 5, 7, 9, 13, 85, 222, 249, 1843
Offset: 1
Examples
m b(m) n a(n) -- ------------------- -- ---- 0 1 1 121 2 12421 1 2 3 1248421 4 124968421 2 4 5 12499368421 3 5 6 1249987368421 7 124999747368421 4 7 8 12499994947368421 9 1249999898947368421 5 9
Programs
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Mathematica
Select[Range[250], PrimeQ[20^# * (5^(# + 1) - 1)/4 + (20^# - 1)/19] &] (* Amiram Eldar, Jan 08 2022 *)
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Python
from sympy import isprime; {print(m, end = ', ') for m in range(2000) if isprime(20**m*(5**(m+1) - 1)//4 + (20**m - 1)//19)}
Comments