The On-Line Encyclopedia of Integer Sequences (OEIS)

A281168 revision #10

A281168

Numbers k such that (25*10^k + 71)/3 is prime.

1, 2, 4, 7, 10, 18, 22, 25, 38, 40, 154, 256, 282, 318, 640, 1095, 1126, 1214, 2588, 5616, 13159, 17644, 18631, 66714, 75465

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OFFSET

1,2

COMMENTS

For k>1, numbers such that the digit 8 followed by k-2 occurrences of the digit 3 followed by the digits 57 is prime (see Example section).

a(26) > 2*10^5.

LINKS

EXAMPLE

2 is in this sequence because (25*10^2 + 71) / 3 = 857 is prime.

Initial terms and primes associated:

a(1) = 1, 107;

a(2) = 2, 857;

a(3) = 4, 83357;

a(4) = 7, 83333357;

a(5) = 10, 83333333357; etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(25*10^# + 71) / 3] &]

CROSSREFS

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jan 16 2017

STATUS

reviewed