A055520 - OEIS

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A055520

Numbers k such that 30*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 3, 4, 5, 6, 7, 17, 39, 49, 59, 77, 100, 150, 318, 381, 783, 1731, 1917, 8854, 11244, 11959, 12129, 18532, 22717, 23364, 24252, 24548, 25323, 30177, 53717, 380975, 424860

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OFFSET

1,2

COMMENTS

Also numbers k such that (10^(k+1)-7)/3 is prime.

REFERENCES

Martin Gardner, Strong Laws of Small Primes, in The Last Recreations, p. 194 (1997).

LINKS

Table of n, a(n) for n=1..33.

Makoto Kamada, Prime numbers of the form 33...331.

Eric Weisstein's World of Mathematics, 3

Index entries for primes involving repunits

FORMULA

a(n) = A055557(n) - 1. - Robert Price, Jan 30 2015

MATHEMATICA

Do[ If[ PrimeQ[30*(10^n - 1)/9 + 1], Print[n]], {n, 0, 50410}]

CROSSREFS

Cf. A055557.

Indices of A033175 that are primes. Cf. A051200, A055557.

Sequence in context: A171678 A039090 A138988 * A037342 A200333 A024644

Adjacent sequences: A055517 A055518 A055519 * A055521 A055522 A055523

KEYWORD

hard,nonn

AUTHOR

Eric W. Weisstein

EXTENSIONS

a(32)-a(33) from Leonid Durman, Jan 09-10 2012

STATUS

approved