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 A034812 Concatenation of 'prevprime(k) and k and prevprime(k)' is a prime. 1
 5, 8, 9, 12, 14, 17, 19, 23, 25, 28, 31, 33, 38, 39, 41, 42, 47, 48, 51, 60, 61, 62, 63, 69, 71, 72, 75, 77, 78, 80, 81, 84, 85, 91, 102, 104, 105, 111, 124, 126, 149, 150, 167, 181, 182, 189, 192, 194, 215, 222, 227, 230, 233, 242, 243, 256, 271, 273, 283, 288, 308 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Andrew Howroyd, Table of n, a(n) for n = 1..10000 EXAMPLE n=222 -> previous prime is 211, thus '211222211' is a prime. MATHEMATICA okQ[n_] := Module[{idppn=IntegerDigits[NextPrime[n, -1]]}, PrimeQ[FromDigits[Join[idppn, IntegerDigits[n], idppn]]]]; Select[Range[350], okQ] (* Harvey P. Dale, Jan 07 2011 *) PROG (PARI) isok(n)=my(x=digits(precprime(n-1))); n>2 && isprime(fromdigits(concat([x, digits(n), x]))) \\ Andrew Howroyd, Aug 13 2024 CROSSREFS Cf. A034808-A034821. Sequence in context: A066812 A100832 A314572 * A260256 A314573 A066467 Adjacent sequences: A034809 A034810 A034811 * A034813 A034814 A034815 KEYWORD nonn,base AUTHOR Patrick De Geest, Oct 15 1998 EXTENSIONS Offset changed by Andrew Howroyd, Aug 13 2024 STATUS approved

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Last modified September 13 20:16 EDT 2024. Contains 375910 sequences. (Running on oeis4.)