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 A285571 Numbers k such that (49*10^k + 383)/9 is prime. 0
 1, 2, 5, 10, 19, 43, 64, 71, 127, 181, 370, 373, 742, 1085, 1171, 1438, 2038, 2269, 2819, 4802, 7742, 12010, 47120, 55129, 139442, 186409 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k>1, numbers such that the digit 5 followed by k-2 occurrences of the digit 4 followed by the digits 87 is prime (see Example section). a(27) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 54w87. EXAMPLE 4 is in this sequence because (49*10^5 + 383)/9 = 544487 is prime. Initial terms and primes associated: a(1) = 1, 97; a(2) = 2, 587; a(3) = 5, 544487; a(4) = 10, 54444444487; a(5) = 19, 54444444444444444487; etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[(49*10^# + 383)/9] &] CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A304973 A016029 A018327 * A000099 A039690 A243938 Adjacent sequences:  A285568 A285569 A285570 * A285572 A285573 A285574 KEYWORD nonn,more,hard AUTHOR Robert Price, Apr 21 2017 EXTENSIONS a(25)-a(26) from Robert Price, Apr 18 2019 STATUS approved

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Last modified August 14 01:55 EDT 2020. Contains 336476 sequences. (Running on oeis4.)