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 A077221 a(0) = 0 and then alternately even and odd numbers in increasing order such that the sum of any two successive terms is a square. 31
 0, 1, 8, 17, 32, 49, 72, 97, 128, 161, 200, 241, 288, 337, 392, 449, 512, 577, 648, 721, 800, 881, 968, 1057, 1152, 1249, 1352, 1457, 1568, 1681, 1800, 1921, 2048, 2177, 2312, 2449, 2592, 2737, 2888, 3041, 3200, 3361, 3528, 3697, 3872, 4049, 4232 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS This sequence arises from reading the line from 0, in the direction 0, 1,... and the same line from 0, in the direction 0, 8,..., in the square spiral whose vertices are the triangular numbers A000217. Cf. A139591, etc. - Omar E. Pol, May 03 2008 The general formula for alternating sums of powers of odd integers is in terms of the Swiss-Knife polynomials P(n,x) A153641 (P(n,0)-(-1)^k*P(n,2*k))/2. Here n=2, thus a(k) = |(P(2,0)-(-1)^k*P(2,2*k))/2|. - Peter Luschny, Jul 12 2009 Axis perpendicular to A046092 in the square spiral whose vertices are the triangular numbers A000217. See the comment above. - Omar E. Pol, Sep 14 2011 Column 8 of A195040. - Omar E. Pol, Sep 28 2011 Concentric octagonal numbers. A139098 and A069129 interleaved. - Omar E. Pol, Sep 17 2011 Subsequence of A194274. - Bruno Berselli, Sep 22 2011 Partial sums of A047522. - Reinhard Zumkeller, Jan 07 2012 Alternating sum of the first n odd squares in decreasing order, n >= 1. Also number of "ON" cells at n-th stage in simple 2-dimensional cellular automaton. The rules are: on the infinite square grid, start with all cells OFF, so a(0) = 0. Turn a single cell to the ON state, so a(1) = 1. At each subsequent step, the neighbor cells of each cell of the old generation are turned ON, and the cells of the old generation are turned OFF. Here "neighbor" refers to the eight adjacent cells of each ON cell. See example. - Omar E. Pol, Feb 16 2014 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..10000 Bruno Berselli, An origin of A077221 (illustration) (see Pol's comment). N. J. A. Sloane, Catalog of Toothpick and Cellular Automata Sequences in the OEIS Index entries for linear recurrences with constant coefficients, signature (2,0,-2,1). FORMULA a(2n) = 8*n^2, a(2n+1) = 8*n(n+1)+1. a(n) = 2*n^2+4*n+1 [+1 if n is odd] with a(0)=1. G.f.: x*(x^2+6*x+1)/(1-x)^3/(1+x). - Ralf Stephan, Mar 31 2003 Row sums of triangle A131925; binomial transform of (1, 7, 2, 4, -8, 16, -32,...). - Gary W. Adamson, Jul 29 2007 a(n) = a(-n); a(n+1) = A195605(n)-(-1)^n. - Bruno Berselli, Sep 22 2011 a(n) = 2*n^2+((-1)^n-1)/2. - Omar E. Pol, Sep 28 2011 EXAMPLE From Omar E. Pol, Feb 16 2014: (Start) Illustration of initial terms as a cellular automaton: . .                                   O O O O O O O .                     O O O O O     O           O .           O O O     O       O     O   O O O   O .     O     O   O     O   O   O     O   O   O   O .           O O O     O       O     O   O O O   O .                     O O O O O     O           O .                                   O O O O O O O . .     1       8           17              32 . (End) MAPLE a := n -> 2*n^2 - (n mod 2); # Peter Luschny, Jul 12 2009 MATHEMATICA a=1; lst={a}; Do[b=n^2-a; AppendTo[lst, b]; a=b, {n, 3, 6!, 2}]; lst (* Vladimir Joseph Stephan Orlovsky, May 18 2009 *) PROG (MAGMA) [2*n^2 - (n mod 2): n in [0..50]]; // Vincenzo Librandi, Sep 22 2011 (Haskell) a077221 n = a077221_list !! n a077221_list = scanl (+) 0 a047522_list -- Reinhard Zumkeller, Jan 07 2012 CROSSREFS Cf. A077222, A131925, A032528, A195041, A195042, A195142. Sequence in context: A247117 A099358 A077222 * A226601 A106648 A209376 Adjacent sequences:  A077218 A077219 A077220 * A077222 A077223 A077224 KEYWORD nonn,easy AUTHOR Amarnath Murthy, Nov 03 2002 EXTENSIONS Extended by Ralf Stephan, Mar 31 2003 STATUS approved

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Last modified October 16 11:23 EDT 2019. Contains 328056 sequences. (Running on oeis4.)