A270831 - OEIS

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

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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.

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 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

Cf. A056654, A268448, A269303, A270339, A270613.

Sequence in context: A048459 A203615 A273932 * A063738 A063737 A338121

Adjacent sequences: A270828 A270829 A270830 * A270832 A270833 A270834

KEYWORD

nonn

AUTHOR

Robert Price, Mar 23 2016

EXTENSIONS

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

STATUS

approved