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 A215421 Primes that remain prime when a single digit 9 is inserted between any two consecutive digits or as the leading or trailing digit. 27
 7, 19, 37, 41, 199, 239, 311, 587, 661, 941, 967, 1009, 1997, 4993, 4999, 5393, 5651, 6911, 9109, 9397, 9679, 9829, 19417, 20233, 22549, 27397, 29389, 31387, 39989, 71419, 71569, 90599, 91951, 95369, 97103, 98909, 99023, 160009, 225919, 267389, 313991, 328849 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Harvey P. Dale, Table of n, a(n) for n = 1..100 EXAMPLE 31387 is prime and also 313879, 313897, 313987, 319387, 391387, 931387. MAPLE A215421:=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: A215421(1000, 9); MATHEMATICA Select[Prime[Range[30000]], AllTrue[FromDigits/@Table[Insert[ IntegerDigits[ #], 9, n], {n, IntegerLength[ #]+1}], PrimeQ]&] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, Oct 22 2016 *) CROSSREFS Cf. A215417-A215420 Sequence in context: A298565 A068678 A069833 * A192594 A352243 A031337 Adjacent sequences: A215418 A215419 A215420 * A215422 A215423 A215424 KEYWORD nonn,base AUTHOR Paolo P. Lava, Aug 10 2012 STATUS approved

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Last modified February 7 22:58 EST 2023. Contains 360132 sequences. (Running on oeis4.)