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.
1, 26, 1016, 5136, 39639, 203100, 2729736, 32264250, 42119062, 1065799391, 12444190246
Offset: 1
Examples
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.
Links
- Samuel Harkness, Illustration of terms 1 through 4
- Rémy Sigrist, C++ program
Extensions
a(8)-a(11) from Rémy Sigrist, Sep 27 2022
Comments