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 A195020 Vertex number of a square spiral in which the length of the first two edges are the legs of the primitive Pythagorean triple [3, 4, 5]. The edges of the spiral have length A195019. 28
 0, 3, 7, 13, 21, 30, 42, 54, 70, 85, 105, 123, 147, 168, 196, 220, 252, 279, 315, 345, 385, 418, 462, 498, 546, 585, 637, 679, 735, 780, 840, 888, 952, 1003, 1071, 1125, 1197, 1254, 1330, 1390, 1470, 1533, 1617, 1683, 1771, 1840, 1932, 2004, 2100 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Zero together with the partial sums of A195019. The spiral contains infinitely many Pythagorean triples in which the hypotenuses on the main diagonal are the positives A008587. The vertices on the main diagonal are the numbers A024966 = (3+4)*A000217 = 7*A000217, where both 3 and 4 are the first two edges in the spiral. The distance "a" between nearest edges that are perpendicular to the initial edge of the spiral is 3, while the distance "b" between nearest edges that are parallel to the initial edge is 4, so the distance "c" between nearest vertices on the same axis is 5 because from the Pythagorean theorem we can write c = (a^2+b^2)^(1/2) = sqrt(3^2+4^2) = sqrt(9+16) = sqrt(25) = 5. Let an array have m(0,n)=m(n,0)=n*(n-1)/2 and m(n,n)=n*(n+1)/2. The first n+1 terms in row(n) are the numbers in the closed interval m(0,n) to m(n,n). The terms in column(n) are the same from m(n,0) to m(n,n). The first few antidiagonals are 0; 0,0; 1,1,1; 3,2,2,3; 6,4,3,4,6; 10,7,5,5,7,10. a(n) is the difference between the sum of the terms in the n+1 X n+1 matrices and those in the n X n matrices.  - J. M. Bergot, Jul 05 2013 [The first five rows are: 0,0,1,3,6; 0,1,2,4,7; 1,2,3,5,8; 3,4,5,6,9; 6,7,8,9,10] LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..10000 Ron Knott, Pythagorean triangles and triples Eric Weisstein's World of Mathematics, Pythagorean Triple Index entries for linear recurrences with constant coefficients, signature (1,2,-2,-1,1). FORMULA From Bruno Berselli, Oct 13 2011:  (Start)   G.f.: x*(3+4*x)/((1+x)^2*(1-x)^3).   a(n) = (1/2)*A004526(n+2)*A047335(n+1) = (2*n*(7*n+13)+(2*n-5)*(-1)^n+5)/16.   a(n) = a(n-1)+2*a(n-2)-2*a(n-3)-a(n-4)+a(n-5).   a(n)-a(n-2) = A047355(n+1). (End) PROG (MAGMA) [(2*n*(7*n+13)+(2*n-5)*(-1)^n+5)/16: n in [0..50]]; // Vincenzo Librandi, Oct 14 2011 CROSSREFS Cf. A024966, A008585, A008586, A001106, A022264, A024966, A033572, A144555, A158482, A158485, A195018, A195032, A195034, A195036. Sequence in context: A025721 A298267 A235532 * A169627 A102508 A115298 Adjacent sequences:  A195017 A195018 A195019 * A195021 A195022 A195023 KEYWORD nonn,easy AUTHOR Omar E. Pol, Sep 07 2011 - Sep 12 2011 STATUS approved

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Last modified November 21 16:04 EST 2019. Contains 329371 sequences. (Running on oeis4.)