login
A272929
Numbers k such that (8*10^k - 77)/3 is prime.
0
2, 4, 5, 6, 15, 18, 43, 45, 55, 60, 105, 128, 180, 207, 271, 479, 869, 1220, 1478, 1937, 4003, 4213, 5503, 9562, 11388, 13120, 34049, 47178, 156371, 271039
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 6 followed by the digits 41 is prime (see Example section).
a(31) > 3*10^5.
EXAMPLE
4 is in this sequence because (8*10^4 - 77)/3 = 26641 is prime.
Initial terms and associated primes:
a(1) = 2, 241;
a(2) = 4, 26641;
a(3) = 5, 266641;
a(4) = 6, 2666641;
a(5) = 15, 2666666666666641, etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(8*10^# - 77)/3] &]
PROG
(PARI) lista(nn) = {for(n=1, nn, if(ispseudoprime((8*10^n - 77)/3), print1(n, ", "))); } \\ Altug Alkan, May 11 2016
KEYWORD
nonn,more
AUTHOR
Robert Price, May 10 2016
EXTENSIONS
a(29) from Robert Price, Jul 07 2018
a(30) from Robert Price, Jul 02 2023
STATUS
approved