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 A195322 a(n) = 20*n^2. 10
 0, 20, 80, 180, 320, 500, 720, 980, 1280, 1620, 2000, 2420, 2880, 3380, 3920, 4500, 5120, 5780, 6480, 7220, 8000, 8820, 9680, 10580, 11520, 12500, 13520, 14580, 15680, 16820, 18000, 19220, 20480, 21780, 23120, 24500, 25920, 27380, 28880, 30420, 32000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Sequence found by reading the line from 0, in the direction 0, 20, ..., in the square spiral whose vertices are the generalized dodecagonal numbers A195162. Semiaxis opposite to A195317 in the same spiral. a(n) is the sum of all the integers less than 10*n which are not multiple of 2 or 5. a(2) = (1 + 3 + 7 + 9) + (11 + 13 + 17 + 19) = 20 + 60 = 80 = 20 * 2^2. (Link Crux Mathematicorum). - Bernard Schott, May 15 2017 For n>1, a(n) is the seventh least number k = x + y, with x>0 and y>0, such that there are n different pairs (x,y) for which x*y/k is an integer. - Paolo P. Lava, Jan 30 2018 Number of terms less than 10^k (k=0, 1, 2, ...): 1, 1, 3, 8, 23, 71, 224, 708, 2237, 7072, 22361, 70711, ... - Muniru A Asiru, Feb 01 2018 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..10000 Léo Sauvé, Problem 53, Crux Mathematicorum, page 88, Vol.1, Nov. 75. Index entries for linear recurrences with constant coefficients, signature (3, -3, 1). FORMULA a(n) = 20*A000290(n) = 10*A001105(n) = 5*A016742(n) = 4*A033429(n) = 2*A033583(n). a(0)=0, a(1)=20, a(2)=80; for n>2, a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3). - Harvey P. Dale, Jan 18 2013 a(n) = A010014(n) - A005899(n) for n>0. - R. J. Cano, Sep 29 2015 EXAMPLE From Muniru A Asiru, Feb 01 2018: (Start) n=0, a(0) = 20*0^2 = 0. n=1, a(1) = 20*1^2 = 20. n=1, a(2) = 20*2^2 = 80. n=1, a(3) = 20*3^2 = 180. n=1, a(4) = 20*4^2 = 320. ... (End) MAPLE a := n -> 20*n^2; seq(a(n), n=0..10^3); # Muniru A Asiru, Feb 01 2018 MATHEMATICA 20 Range[0, 40]^2 (* or *) LinearRecurrence[{3, -3, 1}, {0, 20, 80}, 50] (* Harvey P. Dale, Jan 18 2013 *) PROG (MAGMA) [20*n^2: n in [0..40]]; // Vincenzo Librandi, Sep 20 2011 (PARI) a(n) = 20*n^2 \\ Charles R Greathouse IV, Oct 07 2015 (GAP) List([0..10^3], n->20*n^2); # Muniru A Asiru, Feb 01 2018 CROSSREFS Bisection of A195148. Cf. A033581, A139098, A033583, A135453, A144555, A016802, A195321, A195323. Sequence in context: A211613 A292360 A002609 * A200424 A211463 A244449 Adjacent sequences:  A195319 A195320 A195321 * A195323 A195324 A195325 KEYWORD nonn,easy AUTHOR Omar E. Pol, Sep 16 2011 STATUS approved

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Last modified July 9 13:27 EDT 2020. Contains 335543 sequences. (Running on oeis4.)