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 A006451 Numbers k such that k*(k+1)/2 + 1 is a square. (Formerly M1472) 54
 0, 2, 5, 15, 32, 90, 189, 527, 1104, 3074, 6437, 17919, 37520, 104442, 218685, 608735, 1274592, 3547970, 7428869, 20679087, 43298624, 120526554, 252362877, 702480239, 1470878640, 4094354882, 8572908965, 23863649055, 49966575152 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 REFERENCES A. J. Gottlieb, How four dogs meet in a field, etc., Technology Review, Problem J/A2, Jul/August 1973 pp. 73-74; solution Jan 1974 (see link). Jeffrey Shallit, personal communication. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Reinhard Zumkeller, Table of n, a(n) for n = 0..1000 A. J. Gottlieb, How four dogs meet in a field, etc. (scanned copy) Simon Plouffe, Approximations de séries génératrices et quelques conjectures, Dissertation, Université du Québec à Montréal, 1992; arXiv:0911.4975 [math.NT], 2009. Simon Plouffe, 1031 Generating Functions, Appendix to Thesis, Montreal, 1992 J. Shallit, Letter to N. J. A. Sloane, Oct. 1975 Hermann Stamm-Wilbrandt, 4 interlaced bisections Index entries for linear recurrences with constant coefficients, signature (1,6,-6,-1,1). FORMULA G.f.: x*(-2-3*x+2*x^2+x^3)/(x-1)/(x^2+2*x-1)/(x^2-2*x-1). Conjectured (correctly) by Simon Plouffe in his 1992 dissertation. a(n) = 6*a(n-2) - a(n-4) + 2 with a(0)=0, a(1)=2, a(2)=5, a(3)=15. - Zak Seidov, Apr 15 2008 a(n) = 3*a(n-2) + 4*sqrt((a(n-2)^2 + a(n-2))/2 + 1) + 1 with a(0) = 0, a(1) = 2. - Raphie Frank, Feb 02 2013 a(n) = a(n-1) + 6*a(n-2) - 6*a(n-3) - a(n-4) + a(n-5); a(0)=0, a(1)=2, a(2)=5, a(3)=15, a(4)=32. - Harvey P. Dale, Jul 17 2013 a(n) = 7*a(n-2) - 7*a(n-4) + a(n-6), for n>5. - Hermann Stamm-Wilbrandt, Aug 26 2014 a(2*n+1) = A098790(2*n+1). - Hermann Stamm-Wilbrandt, Aug 26 2014 a(2*n) = A098586(2*n-1), for n>0. - Hermann Stamm-Wilbrandt, Aug 27 2014 a(n) = 8*sqrt(T(a(n-2)) + 1) + a(n-4) where T(a(n)) = A000217(a(n)), and a(-1) = -1, a(0)=0, a(1)=2, a(2)=5. - Vladimir Pletser, Apr 29 2017 MAPLE N:= 100: # to get a(0) to a(N) A[0]:= 0: A[1]:= 2: A[2]:= 5: A[3]:= 15: for n from 4 to N do A[n]:= 6*A[n-2] - A[n-4] + 2 od: seq(A[n], n=0..N); # Robert Israel, Aug 26 2014 MATHEMATICA LinearRecurrence[{1, 6, -6, -1, 1}, {0, 2, 5, 15, 32}, 30] (* Harvey P. Dale, Jul 17 2013 *) Select[Range[10^6], IntegerQ@ Sqrt[# (# + 1)/2 + 1] &] (* Michael De Vlieger, Apr 25 2017 *) PROG (PARI) for(n=1, 10000, t=n*(n+1)/2+1; if(issquare(t), print1(n, ", "))) \\ Joerg Arndt, Oct 10 2009 (Haskell) a006451 n = a006451_list !! n a006451_list = 0 : 2 : 5 : 15 : map (+ 2)    (zipWith (-) (map (* 6) (drop 2 a006451_list)) a006451_list) -- Reinhard Zumkeller, Jan 10 2012 CROSSREFS Cf. A000124, A006452, A006454, A098586, A098790. Cf. numbers m such that k*A000217(m)+1 is a square: this sequence for k=1; m=0 for k=2; A233450 for k=3; A001652 for k=4; A129556 for k=5; A001921 for k=6. - Bruno Berselli, Dec 16 2013 Sequence in context: A077686 A034499 A299159 * A226103 A000962 A118387 Adjacent sequences:  A006448 A006449 A006450 * A006452 A006453 A006454 KEYWORD nonn,easy AUTHOR EXTENSIONS More terms from Larry Reeves (larryr(AT)acm.org), Feb 07 2001 Edited by N. J. A. Sloane, Oct 24 2009, following discussions by several correspondents in the Sequence Fans Mailing List, Oct 10 2009 STATUS approved

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Last modified July 6 21:05 EDT 2022. Contains 355114 sequences. (Running on oeis4.)