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 A001108 a(n)-th triangular number is a square: a(n+1) = 6*a(n)-a(n-1)+2, with a(0) = 0, a(1) = 1. (Formerly M4536 N1924) 61
 0, 1, 8, 49, 288, 1681, 9800, 57121, 332928, 1940449, 11309768, 65918161, 384199200, 2239277041, 13051463048, 76069501249, 443365544448, 2584123765441, 15061377048200, 87784138523761, 511643454094368, 2982076586042449, 17380816062160328, 101302819786919521 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS b(0)=0, c(0)=1, b(i+1)=b(i)+c(i), c(i+1)=b(i+1)+b(i); then a(i) (the number in the sequence) is 2b(i)^2 if i is even, c(i)^2 if i is odd and b(n)=A000129(n) and c(n)=A001333(n). - Darin Stephenson (stephenson(AT)cs.hope.edu) and Alan Koch For n>1 gives solutions to A007913(2x)=A007913(x+1). - Benoit Cloitre, Apr 07 2002 If (X,X+1,Z) is a Pythagorean triple, then Z-X-1 and Z+X are in the sequence. For n >= 2, a(n) gives exactly the positive integers m such that 1,2,...,m has a perfect median. The sequence of associated perfect medians is A001109. Let a_1,...,a_m be an (ordered) sequence of real numbers, then a term a_k is a perfect median if Sum_{j=1..k-1} a_j = Sum_{j=k+1..m} a_j. See Puzzle 1 in MSRI Emissary, Fall 2005. - Asher Auel (auela(AT)math.upenn.edu), Jan 12 2006 This is the r=8 member of the r-family of sequences S_r(n) defined in A092184 where more information can be found. Also, 1^3+2^3+3^3+...+a(n)^3 = k(n)^4 where k(n) is A001109. - Anton Vrba (antonvrba(AT)yahoo.com), Nov 18 2006 The sequence lists the numbers n for which Sum_{i=0..n}{i} is a perfect square. - Paolo P. Lava, Nov 28 2007 If T_x=y^2 is a triangular number which is also a square, the least both triangular and square number which is greater as T_x is T_(3*x+4*y+1)=(2*x+3*y+1)^2 (W. Sierpiński 1961). - Richard Choulet, Apr 28 2009 The remainder of the division of a(n) by 5 is: 0, 1, 3 or 4. The remainder of the division of a(n) by 7 is: 0 or 1. - Mohamed Bouhamida, Aug 26 2009 Number of units of a(n) belongs to a periodic sequence: 0, 1, 8, 9, 8, 1. The remainder of the division of a(n) by 5 belongs to a periodic sequence: 0, 1, 3, 4, 3, 1. - Mohamed Bouhamida, Sep 01 2009 If (a,b) is a solution of the Diophantine equation 0+1+2+...+x=y^2, then a or (a+1) is a perfect square. If (a,b) is a solution of the Diophantine equation 0+1+2+...+x=y^2, then a or a/8 is a perfect square. If (a,b) and (c,d) are two consecutive solutions of the Diophantine equation 0+1+2+...+x=y^2 with a1 a(n) is the index of the first occurrence of -n in sequence A123737. - Vaclav Kotesovec, Jun 02 2015 For n=2*k, k>0, a(n) is divisible by 8 (deficient), so since all proper divisors of deficient numbers are deficient, then a(n) is deficient. For n=2*k+1, k>0, a(n) is odd.  If a(n) is a prime number, it is deficient; otherwise a(n) has one or two distinct prime factors and is therefore deficient again. sigma(a(5)) = 1723 < 3362 = 2*a(5). In either case, a(n) is deficient. - Muniru A Asiru, Apr 14 2016 The squares of NSW numbers (A008843) interleaved with twice squares from A084703, where A008843(n) = A002315(n)^2 and A084703(n) = A001542(n)^2. Conjecture: Also numbers n such that sigma(n) = A000203(n) and sigma(n-th triangular number) = A074285(n) are both odd numbers. - Jaroslav Krizek, Aug 05 2016 For n > 0, numbers for which the number of odd divisors of both n and of n + 1 is odd. - Gionata Neri, Apr 30 2018 a(n) will be solutions to some (A000217(k) + A000217(k+1))/2. - Art Baker, Jul 16 2019 For n>=2, a(n) is the base for which A058331(A001109(n)) is a length-3 repunit. Example: for n=2, A001109(2)=6 and A058331(6)=73 and 73 in base a(2)=8 is 111. See Grantham and Graves. - Michel Marcus, Sep 11 2020 REFERENCES A. H. Beiler, Recreations in the Theory of Numbers, Dover, NY, 1964, p. 193. L. E. Dickson, History of the Theory of Numbers. Carnegie Institute Public. 256, Washington, DC, Vol. 1, 1919; Vol. 2, 1920; Vol. 3, 1923, see vol. 2, p. 10. M. S. Klamkin, "International Mathematical Olympiads 1978-1985," (Supplementary problem N.T.6) W. Sierpiński, Pythagorean triangles, Dover Publications, Inc., Mineola, NY, 2003, pp. 21-22 MR2002669 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Indranil Ghosh, Table of n, a(n) for n = 0..1304 (terms 0..200 from T. D. Noe) Marco Abrate, Stefano Barbero, Umberto Cerruti, and Nadir Murru, Polynomial sequences on quadratic curves, Integers, Vol. 15, 2015, #A38. I. Adler, Three Diophantine equations - Part II, Fib. Quart., 7 (1969), pp. 181-193. M. A. Asiru, All square chiliagonal numbers, Int J Math Edu Sci Technol, 47:7(2016), 1123-1134. Elwyn Berlekamp and Joe P. Buhler, Puzzle Column, Emissary, MSRI Newsletter, Fall 2005. Problem 1, (6 MB). L. Euler, De solutione problematum diophanteorum per numeros integros, Par. 19 H. G. Forder, A Simple Proof of a Result on Diophantine Approximation, Math. Gaz., 47 (1963), 237-238. Jon Grantham and Hester Graves, The abc Conjecture Implies That Only Finitely Many Cullen Numbers Are Repunits, arXiv:2009.04052 [math.NT], 2020. D. B. Hayes, Calculemus!, American Scientist, 96 (Sep-Oct 2008), 362-366. Refik Keskin and Olcay Karaatli, Some New Properties of Balancing Numbers and Square Triangular Numbers, Journal of Integer Sequences, Vol. 15 (2012), Article #12.1.4 P. Lafer, Discovering the square-triangular numbers, Fib. Quart., 9 (1971), 93-105. Ioana-Claudia Lazăr, Lucas sequences in t-uniform simplicial complexes, arXiv:1904.06555 [math.GR], 2019. MSRI newsletter, Emissary, Fall 2005. Vladimir Pletser, Recurrent Relations for Multiple of Triangular Numbers being Triangular Numbers, arXiv:2101.00998 [math.NT], 2020. Simon Plouffe, Approximations de séries génératrices et quelques conjectures, Dissertation, Université du Québec à Montréal, 1992. Simon Plouffe, 1031 Generating Functions, Appendix to Thesis, Montreal, 1992 B. Polster and M. Ross, Marching in squares, arXiv preprint arXiv:1503.04658 [math.HO], 2015. K. Ramsey, Generalized Proof re Square Triangular Numbers, digest of 2 messages in Triangular_and_Fibonacci_Numbers Yahoo group, May 27, 2005 - Oct 10, 2011. Eric Weisstein's World of Mathematics, Square Triangular Number. Eric Weisstein's World of Mathematics, Triangular Number. H. C. Williams and R. K. Guy, Some Monoapparitic Fourth Order Linear Divisibility Sequences Integers, Volume 12A (2012) The John Selfridge Memorial Volume. Index entries for linear recurrences with constant coefficients, signature (7,-7,1). FORMULA a(0) = 0, a(n+1) = 3*a(n) + 1 + 2*sqrt(2*a(n)*(a(n)+1)). - Jim Nastos, Jun 18 2002 a(n) = floor( (1/4) * (3+2*sqrt(2))^n ). - Benoit Cloitre, Sep 04 2002 a(n) = A001653(k)*A001653(k+n) - A001652(k)*A001652(k+n) -A046090(k)*A046090(k+n). - Charlie Marion, Jul 01 2003 a(n) = A001652(n-1) + A001653(n-1) = A001653(n) - A046090(n) = (A001541(n)-1)/2 = a(-n). - Michael Somos, Mar 03 2004 a(n) = 7*a(n-1) - 7*a(n-2) + a(n-3). - Antonio Alberto Olivares, Oct 23 2003 a(n) = Sum_{r=1..n} 2^(r-1)*binomial(2n, 2r). - Lekraj Beedassy, Aug 21 2004 If n>1, then both A000203[n] and A000203[n+1] are odd numbers: n is either square or twice square. - Labos Elemer, Aug 23 2004 a(n) = (T(n, 3)-1)/2 with Chebyshev's polynomials of the first kind evaluated at x=3: T(n, 3)= A001541(n). - Wolfdieter Lang, Oct 18 2004 G.f.: x*(1+x)/((1-x)*(1-6*x+x^2)). Binet form: a(n)=((3+2*sqrt(2))^n +(3-2*sqrt(2))^n-2)/4. - Bruce Corrigan (scentman(AT)myfamily.com), Oct 26 2002 a(n) = floor(sqrt(2*A001110(n))) = floor(A001109(n)*sqrt(2)) = 2*(A000129(n)^2) - (n mod 2) = A001333(n)^2 - 1 + (n mod 2). - Henry Bottomley, Apr 19 2000, corrected by Eric Rowland, Jun 23 2017 A072221(n) = 3*a(n) + 1. - David Scheers, Dec 25 2006 A028982(a(n))+1=A028982(a(n)+1). - Juri-Stepan Gerasimov, Mar 28 2011 a(n+1)^2+a(n)^2+1=6*a(n+1)*a(n)+2*a(n+1)+2*a(n). - Charlie Marion, Sep 28 2011 a(n) = 2*A001653(m)*A053141(n-m-1) + A002315(m)*A046090(n-m-1) + a(m) with m 0. - Richard R. Forberg, Aug 31 2013 From Peter Bala, Jan 28 2014: (Start) A divisibility sequence: that is, a(n) divides a(n*m) for all n and m. Case P1 = 8, P2 = 12, Q = 1 of the 3-parameter family of linear divisibility sequences found by Williams and Guy. a(2*n+1) = A002315(n)^2 = sum {k = 0..4*n + 1} Pell(n), where Pell(n) = A000129(n). a(2*n) = 1/2*A005319(n)^2 = 8*A001109(n)^2. (2,1) entry of the 2X2 matrix T(n,M), where M = [0, -3; 1, 4] and T(n,x) is the Chebyshev polynomial of the first kind. (End) EXAMPLE a(1) = ((3 + 2*sqrt(2)) + (3 - 2*sqrt(2)) - 2) / 4 = (3 + 3 - 2) / 4 = 4 / 4 = 1; a(2) = ((3 + 2*sqrt(2))^2 + (3 - 2*sqrt(2))^2 - 2) / 4 = (9 + 4*sqrt(2) + 8 + 9 - 4*sqrt(2) + 8 - 2) / 4 = (18 + 16 - 2) / 4 = (34 - 2) / 4 = 32 / 4 = 8, etc. MAPLE A001108:=-(1+z)/(z-1)/(z**2-6*z+1); # Simon Plouffe in his 1992 dissertation, without the leading 0 MATHEMATICA Table[(1/2)(-1 + Sqrt[1 + Expand[8(((3 + 2Sqrt)^n - (3 - 2Sqrt)^n)/(4Sqrt))^2]]), {n, 0, 100}] (* Artur Jasinski, Dec 10 2006 *) Transpose[NestList[{#[], #[], 6#[]-#[]+2}&, {0, 1, 8}, 20]][] (* Harvey P. Dale, Sep 04 2011 *) LinearRecurrence[{7, -7, 1}, {0, 1, 8}, 50] (* Vladimir Joseph Stephan Orlovsky, Feb 12 2012 *) PROG (PARI) a(n)=(real((3+quadgen(32))^n)-1)/2 (PARI) a(n)=(subst(poltchebi(abs(n)), x, 3)-1)/2 (PARI) a(n)=if(n<0, a(-n), (polsym(1-6*x+x^2, n)[n+1]-2)/4) (PARI) x='x+O('x^99); concat(0, Vec(x*(1+x)/((1-x)*(1-6*x+x^2)))) \\ Altug Alkan, May 01 2018 (Haskell) a001108 n = a001108_list !! n a001108_list = 0 : 1 : map (+ 2)    (zipWith (-) (map (* 6) (tail a001108_list)) a001108_list) -- Reinhard Zumkeller, Jan 10 2012 (MAGMA) m:=30; R:=PowerSeriesRing(Integers(), m);  cat Coefficients(R!(x*(1+x)/((1-x)*(1-6*x+x^2)))); // G. C. Greubel, Jul 15 2018 CROSSREFS Cf. A001109, A001110, A007913, A000203, A084301, A001652, A072221. Partial sums of A002315. A000129, A005319. a(n) = A115598(n), n > 0. - Hermann Stamm-Wilbrandt, Jul 27 2014 Sequence in context: A089383 A200660 A028443 * A115598 A261668 A097204 Adjacent sequences:  A001105 A001106 A001107 * A001109 A001110 A001111 KEYWORD nonn,easy,nice AUTHOR EXTENSIONS More terms from Larry Reeves (larryr(AT)acm.org), Apr 19 2000 More terms from Lekraj Beedassy, Aug 21 2004 STATUS approved

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Last modified October 19 09:36 EDT 2021. Contains 348074 sequences. (Running on oeis4.)