A174710 - OEIS

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A174710

Numbers k such that k*10^k + (10^k - 1)/3 is prime.

1, 2, 275, 1556, 36682

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OFFSET

1,2

COMMENTS

No term is a multiple of 3. The decimal expansion of each corresponding prime is n's decimal expansion with n 3's concatenated. Primes and probable primes found by PrimeForm. Primes for 275 and 1556 proved by Primo. No more terms up to 30000.

No more terms up to 50000. - Michael S. Branicky, Oct 11 2024

LINKS

Table of n, a(n) for n=1..5.

EXAMPLE

The numbers 1 and 2 are terms because 13 and 233 are prime.

PROG

(PARI) is(n)=ispseudoprime(n*10^n+(10^n-1)/3) \\ Charles R Greathouse IV, Jun 13 2017

CROSSREFS

Cf. A068817 (n followed by n 1's is prime), A133127 (n followed by n 7's is prime), A174352 (n followed by n 9's is prime).

Sequence in context: A135696 A008775 A237993 * A200168 A215043 A119553

Adjacent sequences: A174707 A174708 A174709 * A174711 A174712 A174713

KEYWORD

more,nonn

AUTHOR

Rick L. Shepherd, Mar 27 2010

EXTENSIONS

a(5) from Michael S. Branicky, Oct 11 2024

STATUS

approved