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 A267707 a(n) = A000217(A000217(n)+1). 0
 1, 3, 10, 28, 66, 136, 253, 435, 703, 1081, 1596, 2278, 3160, 4278, 5671, 7381, 9453, 11935, 14878, 18336, 22366, 27028, 32385, 38503, 45451, 53301, 62128, 72010, 83028, 95266, 108811, 123753, 140185, 158203, 177906, 199396, 222778, 248160, 275653, 305371 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS It is the sequence of triangular numbers (A000217) with progressive gaps that grow as 0,1,2,3, ... (consecutive numbers), by which I mean that the 0,1,2,3, ... consecutive triangular numbers are removed from A000217 to form this sequence. For instance, (1), 6, a triangular number, is missing between 3 and 10, which is the gap with 1 triangular number removed, (2), 15 and 21 (two consecutive triangular numbers) are missing between 10 and 28, which is the gap with 2 triangular numbers removed, and so on. The differences between the consecutive terms of this sequence can be expressed through the sum of cubes of two numbers separated by 2 as (n^3+(n+2)^3)/4, which is the same as A229183, except for the first term in there. The same pattern when applied to squares, A000290(A000290(n)+1), gives A082044(n). Triangular numbers are also linked in a similar manner to A027927(n) = A000217(A000217(n)+2)/3. LINKS Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1). FORMULA a(n) = A000217(A000217(n)+1) = (n*(n+1)/2+1)(n*(n+1)/2+2)/2. a(n) = (n^4+2n^3+7n^2+6n+8)/8  = (n^2+n+2)(n^2+n+4)/8. G.f.: (1-2*x+5*x^2-2*x^3+x^4)/(1-x)^5. - Vincenzo Librandi, Jan 22 2016 a(n) = 5*a(n-1)-10*a(n-2)+10*a(n-3)-5*a(n-4)+a(n-5). - Vincenzo Librandi, Jan 22 2016 EXAMPLE For n=0, a(0)=1*2/2=1. For n=2, a(2)=4*5/2=10. MATHEMATICA S[n_] :=n*(n+1)/2; Table[S[S[n]+1], {n, 0, 50}] Table[(n*(n+1)/2+1)(n*(n+1)/2+2)/2, {n, 0, 50}] Table[(n^4+2*n^3+7*n^2+6*n+8)/8, {n, 0, 50}] CoefficientList[Series[(1 - 2 x + 5 x^2 - 2 x^3 + x^4) / (1 - x)^5, {x, 0, 33}], x] (* or *) LinearRecurrence[{5, -10, 10, -5, 1}, {1, 3, 10, 28, 66}, 50] (* Vincenzo Librandi, Jan 22 2016 *) PROG (PARI) for(n=0, 50, print1((n^4+2*n^3+7*n^2+6*n+8)/8 ", ")) (MAGMA) I:=[1, 3, 10, 28, 66]; [n le 5 select I[n] else 5*Self(n-1)-10*Self(n-2)+10*Self(n-3)-5*Self(n-4)+Self(n-5): n in [1..50]]; // Vincenzo Librandi, Jan 22 2016 CROSSREFS Cf. A000217 (triangular numbers), A229183 (consecutive terms differences), A082044 (related sequence for squares), A027927 (related sequence for triangular numbers). Sequence in context: A325793 A325800 A037167 * A221647 A062431 A034351 Adjacent sequences:  A267704 A267705 A267706 * A267708 A267709 A267710 KEYWORD nonn,easy AUTHOR Waldemar Puszkarz, Jan 19 2016 STATUS approved

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Last modified August 11 23:45 EDT 2020. Contains 336434 sequences. (Running on oeis4.)