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

1,1

Comments

b(m) = Sum_{i=0..2m} 2^(m - |m - i|)*10^i.

a(11) > 5000. - Michael S. Branicky, Jun 07 2022

Links

Table of n, a(n) for n=1..10.

Example

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

Mathematica

Select[Range[250], PrimeQ[20^# * (5^(# + 1) - 1)/4 + (20^# - 1)/19] &] (* Amiram Eldar, Jan 08 2022 *)

Prog

(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)}

Crossrefs

Cf. A002477, A016134, A260802.

Sequence in context: A105771 A193494 A161832 * A078404 A056908 A296344

Adjacent sequences: A350589 A350590 A350591 * A350593 A350594 A350595

Keyword

nonn,base,more

Author

Ya-Ping Lu, Jan 07 2022

Status

approved