

A270831


Numbers k such that (7*10^k + 71)/3 is prime.


497



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
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OFFSET

1,2


COMMENTS

For n>1, numbers such that the digit 2 followed by n2 occurrences of the digit 3 followed by the digits 57 is prime (see Example section).
a(41) > 2*10^5.


LINKS

Table of n, a(n) for n=1..40.
Makoto Kamada, Search for 23w57.


EXAMPLE

3 is in this sequence because (7*10^3 + 71)/3 = 2357 is prime.
Initial terms and primes associated:
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

Cf. A056654, A268448, A269303, A270339, A270613.
Sequence in context: A048459 A203615 A273932 * A063738 A063737 A278342
Adjacent sequences: A270828 A270829 A270830 * A270832 A270833 A270834


KEYWORD

nonn,changed


AUTHOR

Robert Price, Mar 23 2016


EXTENSIONS

a(38)a(40) from Robert Price, May 21 2018


STATUS

approved



