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 A215420 Primes that remain prime when a single digit 7 is inserted between any two consecutive digits or as the leading or trailing digit. 12
 3, 19, 97, 433, 487, 541, 691, 757, 853, 1471, 2617, 2953, 4507, 6481, 7351, 7417, 8317, 13177, 31957, 42457, 46477, 47977, 50077, 59053, 71917, 73897, 74377, 77479, 77743, 77761, 79039, 99103, 175687, 220897, 271177, 360973 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Harvey P. Dale, Table of n, a(n) for n = 1..63 (all terms up to and including the 1 millionth prime) EXAMPLE 59053 is prime as are 590537, 590573, 590753, 597053, 579053, and 759053. MAPLE A215420:=proc(q, x) local a, b, c, d, i, n, ok; for n from 1 to q do a:=ithprime(n); b:=0; while a>0 do b:=b+1; a:=trunc(a/10); od; a:=ithprime(n); ok:=1; for i from 0 to b do c:=a+9*10^i*trunc(a/10^i)+10^i*x; if not isprime(c) then ok:=0; break; fi; od; if ok=1 then print(ithprime(n)); fi; od; end: A215420(1000, 7); MATHEMATICA Select[Prime[Range[31000]], AllTrue[FromDigits/@Table[Insert[ IntegerDigits[ #], 7, n], {n, IntegerLength[#]+1}], PrimeQ]&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Oct 29 2020 *) CROSSREFS Cf. A215417, A215418, A215419, A215421. Sequence in context: A049153 A074361 A126187 * A303542 A294251 A294392 Adjacent sequences: A215417 A215418 A215419 * A215421 A215422 A215423 KEYWORD nonn,base AUTHOR Paolo P. Lava, Aug 10 2012 STATUS approved

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Last modified June 25 18:50 EDT 2024. Contains 373707 sequences. (Running on oeis4.)