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 A072221 a(n) = 6*a(n-1) - a(n-2) + 2, with a(0)=1, a(1)=4. 16
 1, 4, 25, 148, 865, 5044, 29401, 171364, 998785, 5821348, 33929305, 197754484, 1152597601, 6717831124, 39154389145, 228208503748, 1330096633345, 7752371296324, 45184131144601, 263352415571284, 1534930362283105, 8946229758127348, 52142448186480985 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS The product of three consecutive triangular numbers with middle term A000217(m) where m is in this sequence is a square. n is in this sequence just in case the triangle with sides 3,n,n+1 has integer area. Equivalently, n such that 2*(n+2)*(n-1) is a square. - James R. Buddenhagen, Oct 19 2008 Triangular numbers that are equal to a square plus one have this sequence as indices. For example, 25th triangular number is 25*26/2 = 325 = 18^2 + 1. - Tanya Khovanova and Alexey Radul, Aug 08 2009 The triangle with sides 3, a(n), a(n)+1 has area A075848(n) if n>=0. - Michael Somos, Dec 25 2018 Compare with A016064 for integers m with triangles with sides 4, m, m+2 and integer area. - Michael Somos, May 11 2019 LINKS Reinhard Zumkeller, Table of n, a(n) for n = 0..1000 Index entries for linear recurrences with constant coefficients, signature (7, -7, 1). FORMULA a(n)={3*A001541(n)-1}/2. a(n)=3*A001108(n)+1. - David Scheers, Dec 25 2006 a(n)=-1/2+(3/4)*((3+sqrt(8))^n+(3-sqrt(8))^n) for n>=0. a(n)=floor((3/4)*(3+sqrt(8))^n) for n>0. - Franz Vrabec, Aug 21 2006 G.f.: (1-3x+4x^2)/((1-x)(1-6x+x^2)). - R. J. Mathar, Sep 09 2008 a(n) = a(-n) for all n in Z. - Michael Somos, Dec 25 2018 EXAMPLE For n=1, the triangle (3, 4, 5) has area 6 = A075848(1). For n=2, the triangle (3, 25, 26) has area 36 = A075848(2). - Michael Somos, Dec 25 2018 MATHEMATICA a[n_] := a[n] = 6a[n - 1] - a[n - 2] + 2; a = 1; a = 4; Table[ a[n], {n, 0, 20}] LinearRecurrence[{7, -7, 1}, {1, 4, 25}, 25] (* T. D. Noe, Dec 09 2013 *) a[ n_] := (3 ChebyshevT[ n, 3] - 1) / 2; (* Michael Somos, Dec 25 2018 *) PROG (Haskell) a072221 n = a072221_list !! n a072221_list = 1 : 4 : (map (+ 2) \$    zipWith (-) (map (* 6) \$ tail a072221_list) a072221_list) -- Reinhard Zumkeller, Apr 27 2012 (PARI) {a(n) = (3 * polchebyshev( n, 1, 3) - 1) / 2}; /* Michael Somos, Dec 25 2018 */ CROSSREFS Cf. A000217, A001108, A001541, A016064, A075848. Sequence in context: A275177 A079291 A173612 * A302064 A221849 A055846 Adjacent sequences:  A072218 A072219 A072220 * A072222 A072223 A072224 KEYWORD nonn,easy AUTHOR Lekraj Beedassy, Jul 04 2002 EXTENSIONS Edited by Robert G. Wilson v, Jul 08 2002 STATUS approved

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Last modified September 26 01:25 EDT 2020. Contains 337346 sequences. (Running on oeis4.)