A299383 - OEIS

A299383

Numbers k such that k * 20^k - 1 is prime.

1, 18, 44, 60, 80, 123, 429, 1166, 2065, 8774, 35340, 42968, 50312, 210129

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OFFSET

1,2

COMMENTS

a(15) > 400000.

LINKS

Table of n, a(n) for n=1..14.

Steven Harvey, Generalized Woodall Search

Wikipedia, Woodall number

MATHEMATICA

Select[Range[1, 10000], PrimeQ[n*20^n-1] &]

PROG

(PARI) for(n=1, 10000, if(isprime(n*20^n-1), print1(n", ")))

(Magma) [n: n in [1..10000] |IsPrime(n*20^n-1)];

CROSSREFS

Numbers n such that n * b^n - 1 is prime: A008864 (b=1), A002234 (b=2), A006553 (b=3), A086661 (b=4), A059676 (b=5), A059675 (b=6), A242200 (b=7), A242201 (b=8), A242202 (b=9), A059671 (b=10), A299374 (b=11), A299375 (b=12), A299376 (b=13), A299377 (b=14), A299378 (b=15), A299379 (b=16), A299380 (b=17), A299381 (b=18), A299382 (b=19), this sequence (b=20).

Sequence in context: A297428 A049067 A114814 * A177724 A095739 A055577

Adjacent sequences: A299380 A299381 A299382 * A299384 A299385 A299386

KEYWORD

nonn,more,hard

AUTHOR

Tim Johannes Ohrtmann, Feb 08 2018

STATUS

approved

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