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 A357376 The lowest number on Ulam Spiral for which all numbers in the square which is centered at a(n) and spans n-1 spaces in each cardinal direction are nonprime. 2
 1, 26, 1016, 5136, 39639, 203100, 2729736, 32264250, 42119062, 1065799391, 12444190246 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The n-th term creates a (2*n-1) X (2*n-1) square centered at a(n). LINKS Table of n, a(n) for n=1..11. Samuel Harkness, Illustration of terms 1 through 4 Rémy Sigrist, C++ program EXAMPLE For n=2, test squares with 1 layer of numbers around the center number until a square is enclosed with only nonprimes. . 65 64 63 62 61 60 59 58 57 . 66 37 36 35 34 33 32 31 56 . 67 38 17 16 15 14 13 30 55 . 68 39 18 5 4 3 12 29 54 . 69 40 19 6 1 2 11 28 53 . +--------------+ 70 41 20 7 8 9 | 10 27 52 | . | | 71 42 21 22 23 24 | 25 26 51 | . | | 72 43 44 45 46 47 | 48 49 50 | . +--------------+ 73 74 75 76 77 78 79 80 81 . The first number tested, 1, does not work because 2, 3, 5 and 7 are in the 1-layered square and are prime. The first square that works is 26 because 10, 27, 52, 25, 26, 51, 48, 49, and 50 are all nonprime. PROG (C++) See Links section. CROSSREFS Cf. A000040, A115258, A136626, A225073. Sequence in context: A335609 A241871 A160261 * A241874 A330497 A037138 Adjacent sequences: A357373 A357374 A357375 * A357377 A357378 A357379 KEYWORD nonn,more AUTHOR Samuel Harkness, Sep 26 2022 EXTENSIONS a(8)-a(11) from Rémy Sigrist, Sep 27 2022 STATUS approved

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Last modified August 6 03:07 EDT 2024. Contains 374957 sequences. (Running on oeis4.)