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 A113688 Isolated semiprimes in the semiprime square spiral. 9
 65, 74, 249, 295, 309, 355, 422, 511, 545, 667, 669, 758, 926, 943, 979, 998, 1099, 1167, 1186, 1322, 1457, 1469, 1561, 1585, 1658, 1711, 1774, 1779, 1835, 1891, 1959, 1961, 1963, 2021, 2038, 2066, 2155, 2186, 2191, 2206, 2271, 2329, 2342 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Write the integers 1, 2, 3, 4, ... in a counterclockwise square spiral. Analogous to Ulam's marking the primes in the spiral and discovering unexpectedly many connected diagonals, we construct a semiprime spiral by marking the semiprimes (A001358). Each integer has 8 adjacent integers in the spiral, horizontally, vertically and diagonally. Curious extended clumps coagulate, slightly denser towards the origin, of semiprimes connected by adjacency. This sequence lists the isolated semiprimes in the semiprime spiral, namely those semiprimes none of whose adjacent integers in the spiral are semiprimes. A113689 gives an enumeration of the number of semiprimes in clumps of size > 1 through n^2. The squares of twin primes occupy adjacent points along the southeast diagonal, so none are isolated. Thus the only isolated semiprimes in the spiral that are squares are the squares of "isolated primes" (A007510). The first square in this sequence is a(1473) = 66049 = 257^2. - Jon E. Schoenfield, Aug 12 2018 REFERENCES S. M. Ellerstein, The square spiral, J. Recreational Mathematics 29 (#3, 1998) 188; 30 (#4, 1999-2000), 246-250. LINKS Michael De Vlieger, Table of n, a(n) for n = 1..2000 Alois P. Heinz, Plot of semiprime spiral, containing all semiprimes <= 10000. Isolated semiprimes are colored red. M. Stein and S. M. Ulam, An Observation on the Distribution of Primes, Amer. Math. Monthly 74, 43-44, 1967. M. Stein and S. M. Ulam and M. B. Wells, A Visual Display of Some Properties of the Distribution of Primes, Amer. Math. Monthly 71, 516-520, 1964. Eric Weisstein's World of Mathematics, Prime Spiral. Eric Weisstein's World of Mathematics, Semiprime. EXAMPLE Spiral example: .   17--16--15--14--13    |               |   18   5---4---3  12    |   |       |   |   19   6   1---2  11    |   |           |   20   7---8---9--10    |   21--22--23--24--25 . From Michael De Vlieger, Dec 22 2015: (Start) Spiral including n <= 121 showing only semiprimes; the isolated semiprimes appear in parentheses: .     .---.---.---.---.---.--95--94--93---.--91     |                                       |     . (65)--.---.--62---.---.---.--58--57   .     |   |                               |   |     .   .   .---.--35--34--33---.---.   .   .     |   |   |                       |   |   |     .   .  38   .---.--15--14---.   .  55   .     |   |   |   |               |   |   |   |     .   .  39   .   .---4---.   .   .   .  87     |   |   |   |   |       |   |   |   |   |   106  69   .   .   6   .---.   .   .   .  86     |   |   |   |   |           |   |   |   |     .   .   .   .   .---.---9--10   .   .  85     |   |   |   |                   |   |   |     .   .   .  21--22---.---.--25--26  51   .     |   |   |                           |   |     .   .   .---.---.--46---.---.--49---.   .     |   |                                   |     .   .-(74)--.---.--77---.---.---.---.--82     |   111---.---.---.-115---.---.-118-119---.-121 . (End) MATHEMATICA spiral[n_] := Block[{o = 2 n - 1, t, w}, t = Table[0, {o}, {o}]; t = ReplacePart[t, {n, n} -> 1]; Do[w = Partition[Range[(2 (# - 1) - 1)^2 + 1, (2 # - 1)^2], 2 (# - 1)] &@ k; Do[t = ReplacePart[t, {(n + k) - (j + 1), n + (k - 1)} -> #[[1, j]]]; t = ReplacePart[t, {n - (k - 1), (n + k) - (j + 1)} -> #[[2, j]]]; t = ReplacePart[t, {(n - k) + (j + 1), n - (k - 1)} -> #[[3, j]]]; t = ReplacePart[t, {n + (k - 1), (n - k) + (j + 1)} -> #[[4, j]]], {j, 2 (k - 1)}] &@ w, {k, 2, n}]; t]; f[w_] := Block[{d = Dimensions@ w, t, g}, t = Reap[Do[Sow@ Take[#[[k]], {2, First@ d - 1}], {k, 2, Last@ d - 1}]][[-1, 1]] &@ w; g[n_] := If[n != 0, Total@ Join[Take[w[[Last@ # - 1]], {First@ # - 1, First@ # + 1}], {First@ #, Last@ #} &@ Take[w[[Last@ #]], {First@ # - 1, First@ # + 1}], Take[w[[Last@ # + 1]], {First@ # - 1, First@# + 1}]] &@(Reverse@ First@ Position[t, n] + {1, 1}) == 0, False]; Select[Union@ Flatten@ t, g@ # &]]; t = spiral@ 26 /. n_ /; PrimeOmega@ n != 2 -> 0; f@ t (* Michael De Vlieger, Dec 21 2015, Version 10 *) CROSSREFS Cf. A001107, A001358, A002939, A002943, A004526, A005620, A007742, A033951-A033954, A033988, A033989-A033991, A033996, A063826. Cf. A115258 (isolated primes in Ulam's lattice). Sequence in context: A095523 A282113 A060877 * A214484 A159758 A056693 Adjacent sequences:  A113685 A113686 A113687 * A113689 A113690 A113691 KEYWORD easy,nonn AUTHOR Jonathan Vos Post, Nov 05 2005 EXTENSIONS Corrected and extended by Alois P. Heinz, Jan 02 2011 STATUS approved

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Last modified December 6 04:14 EST 2019. Contains 329784 sequences. (Running on oeis4.)