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A091823 a(n) = 2*n^2 + 3*n - 1. 17
4, 13, 26, 43, 64, 89, 118, 151, 188, 229, 274, 323, 376, 433, 494, 559, 628, 701, 778, 859, 944, 1033, 1126, 1223, 1324, 1429, 1538, 1651, 1768, 1889, 2014, 2143, 2276, 2413, 2554, 2699, 2848, 3001, 3158, 3319, 3484, 3653, 3826, 4003, 4184, 4369, 4558 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(n) = position in row of Pascal's triangle (A007318) in which three consecutive entries appear in the ratio n: n+1: n+2. (Even valid for n = 0 if you allow for a position of -1 to have value 0.) The solution is unique for each n.

The row numbers are given by A060626.

This sequence plus 1 (i.e., a(n) = 2*n^2 + 3*n) is the sequence A014106. - Howard A. Landman, Mar 28 2004

If Y and Z are a 2-blocks of a 2n-set X then, for n>=2, a(n-1) is the number of (2n-2)-subsets of X intersecting Y. - Milan Janjic, Nov 18 2007

One might prepend an initial -1: "-1, 4, 13, 26, 43, ..." - Vladimir Joseph Stephan Orlovsky, Oct 25 2008 (This would require too many other changes. - N. J. A. Sloane, Mar 27 2014)

LINKS

Bruno Berselli, Table of n, a(n) for n = 1..1000

Guo-Niu Han, Enumeration of Standard Puzzles

Guo-Niu Han, Enumeration of Standard Puzzles, p. 21. [Cached copy]

Milan Janjic, Two Enumerative Functions

Index entries for linear recurrences with constant coefficients, signature (3,-3,1).

FORMULA

a(n) = n + 4*binomial(2+n, n), with offset 0. - Zerinvary Lajos, May 12 2006

G.f.: x*(4 + x - x^2)/(1 - x)^3. - Vincenzo Librandi, Mar 28 2014

EXAMPLE

Entries in the ratio 1:2:3 appear in row 14 starting at position 4 (1001, 2002, 3003); entries in the ratio 2:3:4 appear in row 34 starting at position 13 (927983760, 1391975640, 1855967520); and so on (row 62 pos 26, row 98 pos 43, ...).

MAPLE

A091823:=n->2*n^2 + 3*n - 1; seq(A091823(n), n=1..100); # Wesley Ivan Hurt, Mar 27 2014

MATHEMATICA

Table[2 n^2 + 3 n - 1, {n, 50}] (* Bruno Berselli, Mar 28 2014 *)

PROG

(Perl) #!/usr/bin/perl $a = 1; while (1) { $k = $a*(2*$a + 3) - 1; print "$k, "; $a ++; }

(MAGMA) [2*n^2+3*n-1: n in [1..50]]; // Bruno Berselli, Mar 28 2014

(PARI) a(n)=2*n^2+3*n-1 \\ Charles R Greathouse IV, Sep 24 2015

CROSSREFS

Cf. A007318, A014106, A060626.

Sequence in context: A056708 A307271 A092484 * A024834 A143867 A024809

Adjacent sequences:  A091820 A091821 A091822 * A091824 A091825 A091826

KEYWORD

nonn,easy

AUTHOR

Howard A. Landman, Mar 08 2004

STATUS

approved

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Last modified December 16 01:32 EST 2019. Contains 330013 sequences. (Running on oeis4.)