A176090 Numbers n such that 2(10^n-1)/3 * 10^ceiling(log_10(n+1)) + n is prime.

1, 7, 41, 2429

Offset

1,2

Comments

No term is a multiple of 2, 3, or 5. The decimal expansion of each corresponding prime (in A176272) is n 6's with n's decimal expansion concatenated. Primes and probable primes found by PrimeForm. Prime for 41 proved by Primo. No more terms up to 30000.

Links

Table of n, a(n) for n=1..4.

Example

The numbers 1 and 7 are terms because 61 and 66666667 are prime.

Prog

(PARI) is(n)=ispseudoprime(2*(10^n-1)/3 * 10^logint(10*n, 10) + n) \\ Charles R Greathouse IV, May 22 2017

Crossrefs

Cf. A176272 (corresponding primes), n k's followed by n is prime: A070746 (k=1), A176087 (k=3), A176089 (k=4), A084428 (k=7), A176091 (k=9). [k=2, 5, and 8 produce only composites divisible by 3.]

Sequence in context: A002701 A057006 A144747 * A183064 A290045 A188066

Adjacent sequences: A176087 A176088 A176089 * A176091 A176092 A176093

Keyword

more,nonn

Author

Rick L. Shepherd, Apr 13 2010

Status

approved