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Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!) A032684 Numbers k such that k surrounded by digit '7' is prime. 6 2, 5, 8, 9, 12, 17, 18, 20, 23, 24, 29, 30, 41, 45, 47, 48, 50, 51, 53, 54, 57, 60, 68, 71, 72, 75, 81, 86, 87, 90, 92, 93, 114, 116, 123, 125, 128, 131, 132, 134, 138, 143, 152, 153, 159, 164, 170, 177, 180, 183, 186, 188, 191, 194, 198, 204, 207 (list; graph; refs; listen; history; text; internal format) OFFSET 1,1 LINKS Daniel Starodubtsev, Table of n, a(n) for n = 1..10000 PROG (Python) from sympy import isprime print([i for i in range(200) if isprime(int('7' + str(i) + '7'))]) # Daniel Starodubtsev, Apr 05 2020 CROSSREFS Cf. A032689. Sequence in context: A045928 A190768 A184866 * A026448 A258353 A173286 Adjacent sequences: A032681 A032682 A032683 * A032685 A032686 A032687 KEYWORD nonn,base AUTHOR Patrick De Geest, May 15 1998 STATUS approved

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Last modified August 19 03:19 EDT 2024. Contains 375284 sequences. (Running on oeis4.)