OFFSET
0,6
COMMENTS
The point with coordinates (x = A025581(n), y = A002262(n)) sweeps out the first quadrant by upwards antidiagonals. N. J. A. Sloane, Jul 17 2018
Old name: Integers 0 to n followed by integers 0 to n+1 etc.
a(n) = n - the largest triangular number <= n. - Amarnath Murthy, Dec 25 2001
The PARI functions t1, t2 can be used to read a square array T(n,k) (n >= 0, k >= 0) by antidiagonals downwards: n -> T(t1(n), t2(n)). - Michael Somos, Aug 23 2002
Values x of unique solution pair (x,y) to equation T(x+y) + x = n, where T(k)=A000217(k). - Lekraj Beedassy, Aug 21 2004
Concatenation of the set representation of ordinal numbers, where the n-th ordinal number is represented by the set of all ordinals preceding n, 0 being represented by the empty set. - Daniel Forgues, Apr 27 2011
An integer sequence is nonnegative if and only if it is a subsequence of this sequence. - Charles R Greathouse IV, Sep 21 2011
a(A195678(n)) = A000040(n) and a(m) <> A000040(n) for m < A195678(n), an example of the preceding comment. - Reinhard Zumkeller, Sep 23 2011
A sequence B is called a reluctant sequence of sequence A, if B is triangle array read by rows: row number k coincides with first k elements of the sequence A. A002262 is reluctant sequence of 0,1,2,3,... The nonnegative integers, A001477. - Boris Putievskiy, Dec 12 2012
LINKS
Charles R Greathouse IV, Rows n = 0..100, flattened
Boris Putievskiy, Transformations [Of] Integer Sequences And Pairing Functions, arXiv preprint arXiv:1212.2732 [math.CO], 2012.
Michael Somos, Sequences used for indexing triangular or square arrays
FORMULA
a(n) = A002260(n) - 1.
a(n) = n - (trinv(n)*(trinv(n)-1))/2; trinv := n -> floor((1+sqrt(1+8*n))/2) (cf. A002024); # gives integral inverses of triangular numbers
a(n) = f(n,1) with f(n,m) = if n<m then n else f(n-m,m+1). - Reinhard Zumkeller, May 20 2009
a(n) = (1/2)*(t - t^2 + 2*n), where t = floor(sqrt(2*n+1) + 1/2) = round(sqrt(2*n+1)). - Ridouane Oudra, Dec 01 2019
a(n) = ceiling((-1 + sqrt(9 + 8*n))/2) * (1 - ((1/2)*ceiling((1 + sqrt(9 + 8*n))/2))) + n. - Ryan Jean, Sep 03 2022
G.f.: x*y/((1 - x)*(1 - x*y)^2). - Stefano Spezia, Feb 21 2024
EXAMPLE
From Daniel Forgues, Apr 27 2011: (Start)
Examples of set-theoretic representation of ordinal numbers:
0: {}
1: {0} = {{}}
2: {0, 1} = {0, {0}} = {{}, {{}}}
3: {0, 1, 2} = {{}, {0}, {0, 1}} = ... = {{}, {{}}, {{}, {{}}}} (End)
From Omar E. Pol, Jul 15 2012: (Start)
0;
0, 1;
0, 1, 2;
0, 1, 2, 3;
0, 1, 2, 3, 4;
0, 1, 2, 3, 4, 5;
0, 1, 2, 3, 4, 5, 6;
0, 1, 2, 3, 4, 5, 6, 7;
0, 1, 2, 3, 4, 5, 6, 7, 8;
0, 1, 2, 3, 4, 5, 6, 7, 8, 9;
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10;
(End)
MAPLE
seq(seq(i, i=0..n), n=0..14); # Peter Luschny, Sep 22 2011
A002262 := n -> n - binomial(floor((1/2)+sqrt(2*(1+n))), 2);
MATHEMATICA
m[n_]:= Floor[(-1 + Sqrt[8n - 7])/2]
b[n_]:= n - m[n] (m[n] + 1)/2
Table[m[n], {n, 1, 105}] (* A003056 *)
Table[b[n], {n, 1, 105}] (* A002260 *)
Table[b[n] - 1, {n, 1, 120}] (* A002262 *)
(* Clark Kimberling, Jun 14 2011 *)
Flatten[Table[k, {n, 0, 14}, {k, 0, n}]] (* Alonso del Arte, Sep 21 2011 *)
Flatten[Table[Range[0, n], {n, 0, 15}]] (* Harvey P. Dale, Jan 31 2015 *)
PROG
(PARI) a(n)=n-binomial(round(sqrt(2+2*n)), 2)
(PARI) t1(n)=n-binomial(floor(1/2+sqrt(2+2*n)), 2) /* A002262, this sequence */
(PARI) t2(n)=binomial(floor(3/2+sqrt(2+2*n)), 2)-(n+1) /* A025581, cf. comment by Somos for reading arrays by antidiagonals */
(PARI) concat(vector(15, n, vector(n, i, i-1))) \\ M. F. Hasler, Sep 21 2011
(PARI) apply( {A002262(n)=n-binomial(sqrtint(8*n+8)\/2, 2)}, [0..99]) \\ M. F. Hasler, Oct 20 2022
(Haskell)
a002262 n k = a002262_tabl !! n !! k
a002262_row n = a002262_tabl !! n
a002262_tabl = map (enumFromTo 0) [0..]
a002262_list = concat a002262_tabl
-- Reinhard Zumkeller, Aug 05 2015, Jul 13 2012, Mar 07 2011
(Python)
for i in range(16):
for j in range(i):
print(j, end=", ") # Mohammad Saleh Dinparvar, May 13 2020
(Python)
from math import comb, isqrt
def a(n): return n - comb((1+isqrt(8+8*n))//2, 2)
print([a(n) for n in range(105)]) # Michael S. Branicky, May 07 2023
KEYWORD
AUTHOR
Angele Hamel (amh(AT)maths.soton.ac.uk)
EXTENSIONS
New name from Omar E. Pol, Jul 15 2012
Typo in definition fixed by Reinhard Zumkeller, Aug 05 2015
STATUS
approved