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A014132 T(n,k) = ((n+k)^2 + n-k)/2, n, k > 0, read by antidiagonals. 27
2, 4, 5, 7, 8, 9, 11, 12, 13, 14, 16, 17, 18, 19, 20, 22, 23, 24, 25, 26, 27, 29, 30, 31, 32, 33, 34, 35, 37, 38, 39, 40, 41, 42, 43, 44, 46, 47, 48, 49, 50, 51, 52, 53, 54, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 79 (list; table; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Complement of triangular numbers A000217.

Numbers that are not triangular (nontriangular numbers).

Also definable as follows: a(1)=2; for n>1, a(n) is smallest integer greater than a(n-1) such that the condition "n and a(a(n)) have opposite parities" can always be satisfied. - Benoit Cloitre and Matthew Vandermast, Mar 10 2003

A248952(a(n)) < 0. - Reinhard Zumkeller, Oct 20 2014

Record values in A256188 that are greater than 1: a(n) = A256188(A004202(n)). - Reinhard Zumkeller, Mar 26 2015

Daniel Forgues, Apr 10 2015: (Start)

With n >= 1, k >= 1:

  t(n+k) - k, 1 <= k <= n+k-1, n >= 1;

  t(n+k-1) + n, 1 <= n <= n+k-1, k >= 1;

where t(n+k) = t(n+k-1) + (n+k) is (n+k)-th triangular number, while the number of compositions of n+k into 2 parts is C(n+k-1, 2-1) = n+k-1, the number of nontriangular numbers between t(n+k-1) and t(n+k), just right!

Related to Hilbert's Infinite Hotel:

0) All rooms, numbered through the positive integers, are full;

1) An infinite number of trains, each containing an infinite number of passengers, arrives: i.e., a 2-D lattice of pairs of positive integers;

2) Move occupant of room m, m >= 1, to room t(m) = m*(m+1)/2, where t(m) is the m-th triangular number;

3) Assign n-th passenger from k-th train to room t(n+k-1) + n, 1 <= n <= n+k-1, k >= 1;

4) Everybody has his or her own room, no room is empty, for m >= 1.

If situation 1 happens again, repeat steps 2 and 3, you're back to 4.

(End)

1711 + 2*a(n)*(58 + a(n)) is prime for n<=21. The terms that do not have this property start 29,32,34,43,47,58,59,60,62,63,65,68,70,73,...  - Benedict W. J. Irwin, Nov 22 2016

LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000

B. Cloitre, N. J. A. Sloane and M. J. Vandermast, Numerical analogues of Aronson's sequence, J. Integer Seqs., Vol. 6 (2003), #03.2.2.

B. Cloitre, N. J. A. Sloane and M. J. Vandermast, Numerical analogues of Aronson's sequence arXiv:math/0305308 [math.NT], 2003.

Cristinel Mortici, Remarks on Complementary Sequences, Fibonacci Quart. 48 (2010), no. 4, 343-347.

Boris Putievskiy, Transformations [of] Integer Sequences And Pairing Functions, arXiv:1212.2732 [math.CO], 2012.

FORMULA

a(n) = n + round(sqrt(2*n)).

a(a(n)) = n + 2*floor(1/2 + sqrt(2n)) + 1.

a(n) = a(n-1) + A035214(n), a(1)=2.

a(n) = A080036(n) - 1.

a(n) = n + A002024(n). - Vincenzo Librandi, Jul 08 2010

A010054(a(n)) = 0. - Reinhard Zumkeller, Dec 10 2012

From Boris Putievskiy, Jan 14 2013: (Start)

a(n) = A007401(n)+1.

a(n) = A003057(n)^2 - A114327(n).

a(n) = ((t+2)^2 + i - j)/2, where

i = n-t*(t+1)/2,

j = (t*t+3*t+4)/2-n,

t = floor((-1+sqrt(8*n-7))/2). (End)

From Robert Israel, Apr 20 2015 (Start):

a(n) = A118011(n) - n.

G.f.: x/(1-x)^2 + x/(1-x) * Sum(j>=0, x^(j*(j+1)/2)) = x/(1-x)^2 + x^(7/8)/(2-2*x) * Theta2(0,sqrt(x)), where Theta2 is a Jacobi theta function. (End)

EXAMPLE

From Boris Putievskiy, Jan 14 2013: (Start)

Start of the sequence as a table (read by antidiagonals, right to left), where the k-th row corresponds to the k-th column of the rectangular triangle (shown thereafter):

   2,  4,  7, 11, 16, 22, 29, ...

   5,  8, 12, 17, 23, 30, 38, ...

   9, 13, 18, 24, 31, 39, 48, ...

  14, 19, 25, 32, 40, 49, 59, ...

  20, 26, 33, 41, 50, 60, 71, ...

  27, 34, 42, 51, 61, 72, 84, ...

  35, 43, 52, 62, 73, 85, 98, ...

  (...)

Start of the sequence as a rectangular triangle (read by rows), where the i elements of the i-th row are t(i) + 1 up to t(i+1) - 1, i >= 1:

   2;

   4,  5;

   7,  8,  9;

  11, 12, 13, 14;

  16, 17, 18, 19, 20;

  22, 23, 24, 25, 26, 27;

  29, 30, 31, 32, 33, 34, 35;

  (...)

Row number i contains i numbers, where t(i) = i*(i+1)/2:

  t(i) + 1, t(i) + 2, ..., t(i) + i = t(i+1) - 1

(End) [Edited by Daniel Forgues, Apr 11 2015]

MATHEMATICA

f[n_] := n + Round[Sqrt[2n]]; Array[f, 71] (* or *)

Complement[ Range[83], Array[ #(# + 1)/2 &, 13]] (* Robert G. Wilson v, Oct 21 2005 *)

PROG

(PARI) a(n)=if(n<1, 0, n+(sqrtint(8*n-7)+1)\2)

(PARI) isok(n) = !ispolygonal(n, 3); \\ Michel Marcus, Mar 01 2016

(MAGMA) IsTriangular:=func< n | exists{ k: k in [1..Isqrt(2*n)] | n eq (k*(k+1) div 2)} >; [ n: n in [1..90] | not IsTriangular(n) ]; // Klaus Brockhaus, Jan 04 2011

(Haskell)

a014132 n = n + round (sqrt $ 2 * fromInteger n)

a014132_list = filter ((== 0) . a010054) [0..]

-- Reinhard Zumkeller, Dec 12 2012

CROSSREFS

Cf. A000217, A006002, A035214, A080036, A002024, A007401, A003057, A114327, A002260, A004736, A118011.

Cf. A000124 (left edge: quasi-triangular numbers), A000096 (right edge: almost-triangular numbers), A006002 (row sums), A001105 (central terms).

Cf. A242401 (subsequence).

Cf. A004202, A256188.

Cf. A145397 (the non-tetrahedral numbers).

Sequence in context: A010423 A035235 A253723 * A184008 A183862 A254058

Adjacent sequences:  A014129 A014130 A014131 * A014133 A014134 A014135

KEYWORD

nonn,easy,nice,tabl

AUTHOR

N. J. A. Sloane

EXTENSIONS

Following Alford Arnold's comment: keyword tabl and correspondent crossrefs added by Reinhard Zumkeller, Dec 12 2012

STATUS

approved

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Last modified May 30 06:51 EDT 2017. Contains 287302 sequences.