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A176091
Numbers n such that (10^n-1) * 10^ceiling(log_10(n+1)) + n is prime.
2
307, 1759, 2963, 3881
OFFSET
1,1
COMMENTS
No term is a multiple of 2, 3, or 5. (In fact, a(1) through a(4) are prime.) The decimal expansion of each corresponding prime is n 9's with n's decimal expansion concatenated. Probable primes found by PrimeForm. Primes for 307 and 1759 proved by Primo. No more terms up to 30000.
CROSSREFS
Cf. A174352 (n followed by n 9's is prime), n k's followed by n is prime: A070746 (k=1), A176087 (k=3), A176089 (k=4), A176090 (k=6), A084428 (k=7). [k=2, 5, and 8 produce only composites divisible by 3.]
Sequence in context: A308785 A142572 A204477 * A359640 A118284 A234030
KEYWORD
more,nonn
AUTHOR
Rick L. Shepherd, Apr 16 2010
STATUS
approved