A056707 - OEIS

A056707

Numbers k such that 4*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 16, 31, 37, 55, 62, 172, 174, 197, 727, 1246, 1752, 4318, 4328, 4930, 6450, 15206, 24041, 25321, 33068, 41484, 51511

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OFFSET

1,2

COMMENTS

Also numbers k such that (13*10^k - 1)/3 is prime.

a(24) > 10^5. - Robert Price, Dec 21 2014

LINKS

EXAMPLE

43 and 433 are prime, so 1 and 2 are terms of the sequence.

MATHEMATICA

Do[ If[ PrimeQ[ 4*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}]

CROSSREFS

KEYWORD

hard,nonn

AUTHOR

EXTENSIONS

26044 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(18)-a(23) from Robert Price, Dec 21 2014

STATUS

approved