The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A295083 Numbers k such that (115*10^k - 7)/9 is prime. 0
 1, 2, 14, 26, 53, 86, 133, 206, 245, 323, 362, 872, 4562, 12577, 14294, 18425, 21713, 27728, 87368, 106772 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k > 0, numbers k such that the digits 12 followed by k occurrences of the digit 7 is prime (see Example section). a(21) > 2*10^5. LINKS Table of n, a(n) for n=1..20. Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 127w. EXAMPLE 2 is in this sequence because (115*10^2 - 7)/9 = 1277 is prime. Initial terms and associated primes: a(1) = 1, 127; a(2) = 2, 1277; a(3) = 14, 1277777777777777; a(4) = 26, 1277777777777777777777777777; etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[(115*10^# - 7)/9] &] Select[Range[87500], PrimeQ[FromDigits[PadRight[{1, 2}, #, 7]]]&]-2 (* Harvey P. Dale, May 27 2018 *) PROG (PARI) isok(k) = isprime((115*10^k - 7)/9); \\ Michel Marcus, Nov 14 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A017545 A280268 A109080 * A180877 A032461 A072390 Adjacent sequences: A295080 A295081 A295082 * A295084 A295085 A295086 KEYWORD nonn,more,hard AUTHOR Robert Price, Nov 13 2017 EXTENSIONS a(20) from Robert Price, Feb 08 2020 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified July 16 12:01 EDT 2024. Contains 374348 sequences. (Running on oeis4.)