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A270831
Numbers k such that (7*10^k + 71)/3 is prime.
498
1, 2, 3, 4, 5, 7, 23, 29, 37, 39, 40, 89, 115, 189, 227, 253, 301, 449, 533, 607, 969, 1036, 1207, 1407, 1701, 3493, 7147, 11342, 21638, 22327, 25575, 25648, 34079, 39974, 47719, 49913, 74729, 100737, 103531, 168067
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 3 followed by the digits 57 is prime (see Example section).
a(41) > 2*10^5.
EXAMPLE
3 is in this sequence because (7*10^3 + 71)/3 = 2357 is prime.
Initial terms and associated primes:
a(1) = 1, 47;
a(2) = 2, 257;
a(3) = 3, 2357;
a(4) = 4, 23357;
a(5) = 5, 233357, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(7*10^# + 71)/3] &]
PROG
(PARI) lista(nn) = {for(n=1, nn, if(ispseudoprime((7*10^n + 71)/3), print1(n, ", "))); } \\ Altug Alkan, Mar 23 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert Price, Mar 23 2016
EXTENSIONS
a(38)-a(40) from Robert Price, May 21 2018
STATUS
approved