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 A331864 Numbers k such that R(k) + 2*10^floor(k/2-1) is prime, where R(k) = (10^k-1)/9 (repunit: A002275). 4
 2, 3, 5, 8, 9, 39, 78, 81, 155, 249, 387, 395, 510, 711, 1173, 1751, 10245 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The corresponding primes are near-repunit primes, cf. A105992. In base 10, R(k) + 2*10^floor(k/2-1) has ceiling(k/2) digits 1, one digit 3 and again floor(k/2-1) digits 1: for even as well as odd k, there is a digit 3 just left of the middle of the repunit of length k. No term can be equivalent to 1 (mod 3). - Chai Wah Wu, Feb 07 2020 LINKS Table of n, a(n) for n=1..17. Brady Haran and Simon Pampena, Glitch Primes and Cyclops Numbers, Numberphile video (2015). EXAMPLE For k = 2, R(2) + 2*10^(1-1) = 13 is prime. For k = 3, R(3) + 2*10^(1-1) = 113 is prime. For k = 5, R(5) + 2*10^(2-1) = 11131 is prime. For k = 8, R(8) + 2*10^(4-1) = 11113111 is prime. PROG (PARI) for(n=2, 999, isprime(p=10^n\9+2*10^(n\2-1))&&print1(n", ")) CROSSREFS Cf. A105992 (near-repunit primes), A002275 (repunits), A011557 (powers of 10). Cf. A331865 (variant with floor(n/2) instead of floor(n/2-1)), A331860, A331863 (variants with digit 2 resp. 0 instead of digit 3). Sequence in context: A056903 A229139 A293277 * A272669 A028770 A321702 Adjacent sequences: A331861 A331862 A331863 * A331865 A331866 A331867 KEYWORD nonn,base,hard,more AUTHOR M. F. Hasler, Jan 30 2020 EXTENSIONS a(13)-a(16) from Daniel Suteu, Feb 01 2020 a(17) from Michael S. Branicky, Feb 03 2023 STATUS approved

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Last modified August 10 00:01 EDT 2024. Contains 375044 sequences. (Running on oeis4.)