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 A284191 Numbers k such that 4*10^k - 87 is prime. 0
 2, 5, 7, 13, 16, 20, 29, 37, 43, 49, 101, 888, 2533, 2569, 2599, 2666, 3689, 6335, 9634, 12445, 17669, 51208, 79729 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 9 followed by the digits 13 is prime (see Example section). a(24) > 2*10^5. LINKS Table of n, a(n) for n=1..23. Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 39w13. EXAMPLE 5 is in this sequence because 4*10^5 - 87 = 399913 is prime. Initial terms and associated primes: a(1) = 2, 313; a(2) = 5, 399913; a(3) = 7, 39999913; a(4) = 13, 39999999999913; a(5) = 16, 39999999999999913; etc. MATHEMATICA Select[Range[2, 100000], PrimeQ[4*10^#-87] &] PROG (PARI) isok(n) = isprime(4*10^n - 87); \\ Indranil Ghosh, Mar 22 2017 (Python) from sympy import isprime def ok(n): return 1 if isprime(4*10**n - 87) else 0 # Indranil Ghosh, Mar 22 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A062879 A272193 A186131 * A065897 A293762 A161889 Adjacent sequences: A284188 A284189 A284190 * A284192 A284193 A284194 KEYWORD nonn,more,hard AUTHOR Robert Price, Mar 22 2017 STATUS approved

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Last modified September 15 08:55 EDT 2024. Contains 375932 sequences. (Running on oeis4.)