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 A007401 Add n-1 to n-th term of 'n appears n times' sequence (A002024). (Formerly M2316) 27
 1, 3, 4, 6, 7, 8, 10, 11, 12, 13, 15, 16, 17, 18, 19, 21, 22, 23, 24, 25, 26, 28, 29, 30, 31, 32, 33, 34, 36, 37, 38, 39, 40, 41, 42, 43, 45, 46, 47, 48, 49, 50, 51, 52, 53, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Complement of A000096 = increasing sequence of positive integers excluding n*(n+3)/2. - Jonathan Vos Post, Aug 13 2005 As a triangle: (1; 3,4; 6,7,8; 10,11,12,13; ...), Row sums = A127736: (1, 7, 21, 46, 85, 141, 217, ...). - Gary W. Adamson, Oct 25 2007 Odd primes are a subsequence except 5, cf. A004139. - Reinhard Zumkeller, Jul 18 2011 A023532(a(n)) = 1. - Reinhard Zumkeller, Dec 04 2012 T(n,k) = ((n+k)^2+n-k)/2 - 1, n,k > 0, read by antidiagonals. - Boris Putievskiy, Jan 14 2013 A023531(a(n)) = 0. - Reinhard Zumkeller, Feb 14 2015 REFERENCES 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 = 1..10000 Boris Putievskiy, Transformations [of] Integer Sequences And Pairing Functions arXiv:1212.2732 [math.CO], 2012. T. R. S. Walsh & N. J. A. Sloane, Correspondence, 1991 T. R. S. Walsh, Data (Preprint 1, Part 1) T. R. S. Walsh, Data (Preprint 1, Part 2) T. R. S. Walsh, Data (Preprint 1, Part 3) T. R. S. Walsh, Notes T. R. S. Walsh, Number of sensed planar maps with n edges and m vertices N. C. Wormald, On the number of planar maps, Can. J. Math. 33.1 (1981), 1-11. (Annotated scanned copy) FORMULA From Boris Putievskiy, Jan 14 2013: (Start) a(n) = A014132(n) - 1. a(n) = A003057(n)^2 - A114327(n) - 1. a(n) = ((t+2)^2 + i - j)/2-1, where i = n-t*(t+1)/2, j = (t*t+3*t+4)/2-n, t = floor((-1+sqrt(8*n-7))/2). (End) EXAMPLE From Boris Putievskiy, Jan 14 2013: (Start) The start of the sequence as table:    1,  3,  6, 10, 15, 21, 28, ...    4,  7, 11, 16, 22, 29, 37, ...    8, 12, 17, 23, 30, 38, 47, ...   13, 18, 24, 31, 39, 48, 58, ...   19, 25, 32, 40, 49, 59, 70, ...   26, 33, 41, 50, 60, 71, 83, ...   34, 42, 51, 61, 72, 84, 97, ...   ... The start of the sequence as triangle array read by rows:    1;    3,  4;    6,  7,  8;   10, 11, 12, 13;   15, 16, 17, 18, 19;   21, 22, 23, 24, 25, 26;   28, 29, 30, 31, 32, 33, 34;   ... Row number r contains r numbers r*(r+1)/2, r*(r+1)/2+1, ..., r*(r+1)/2+r-1. (End) MATHEMATICA f[n_] := n + Floor[ Sqrt[2n] - 1/2]; Array[f, 66]; (* Robert G. Wilson v, Feb 13 2011 *) PROG (PARI) a(n)=n+floor(sqrt(n+n)-1/2) \\ Charles R Greathouse IV, Feb 13 2011 (PARI) for(m=1, 9, for(n=m*(m+1)/2, (m^2+3*m-2)/2, print1(n", "))) \\ Charles R Greathouse IV, Feb 13 2011 (Haskell) a007401 n = a007401_list !! n a007701_list = [x | x <- [0..], a023531 x == 0] -- Reinhard Zumkeller, Feb 14 2015, Dec 04 2012 CROSSREFS Cf. A002024, A000096, A127736, A014132, A002260, A004736, A003057, A114327, A023531. Sequence in context: A179872 A075747 A143344 * A275481 A234349 A042954 Adjacent sequences:  A007398 A007399 A007400 * A007402 A007403 A007404 KEYWORD nonn,easy,changed AUTHOR STATUS approved

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