1,2

Old name was: The lone multiple of n among the next n numbers.

Another old name: a(n) = n*floor((n+1)/2).

Consider the triangle

1

2 3

4 5 6

7 8 9 10

11 12 13 14 15

16 17 18 19 20 21

22 23 24 25 26 27 28

... Then sequence contains the multiple of n in the n-th row.

Interleaves A000384 and A001105. - Paul Barry, Jun 29 2006

Termwise products of the natural numbers and the natural numbers repeated.

Number of pairs (x,y) having the same parity, with x in {0,...,n} and y in {0,...,2n}. - Clark Kimberling, Jul 02 2012

Similar to generalized hexagonal numbers A000217. Other members of this family are A210977, A006578, A210978, A181995, A210981, A210982. - Omar E. Pol, Aug 09 2012

For even n, a(n) gives the sum of all the parts in the partitions of n into exactly two parts. For odd n>1, a(n) gives n plus the sum of all the parts in the partitions of n into exactly two parts. - Wesley Ivan Hurt, Nov 14 2013

Number of regions of the plane that do not contain the origin, in the arrangement of lines with polar equations rho = 1/cos(theta-k*2*Pi/n), k=0..n-1; or, by inversion, number of bounded regions in the arrangement of circles with radius 1 and centers the n-th roots of unity. - Luc Rousseau, Feb 08 2019

Numbers k such that floor(sqrt(2k)+1/2) | k. - Wesley Ivan Hurt, Dec 01 2020

a(n) is the maximum size of a subset of cells in an n X n grid such that no two chosen cells are horizontally adjacent and no two chosen cells lie in diagonally adjacent positions in consecutive rows (upper-left/upper-right). Equivalently, it is the independence number of the graph on the n X n grid with edges between horizontal neighbors and between diagonal neighbors in adjacent rows. - Maksim Kita, Dec 06 2025

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Torleiv Kløve, Linear recurring sequences in boolean rings, Math. Scand., 33 (1973), 5-12.

Torleiv Kløve, Linear recurring sequences in boolean rings, Math. Scand., 33 (1973), 5-12. (Annotated scanned copy)

Luc Rousseau, Illustration, a(n) viewed as a number of regions in an arrangement of lines / of circles.

Index entries for linear recurrences with constant coefficients, signature (1,2,-2,-1,1).

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

a(n) = n*A008619(n).

a(2*n-1) = n*(2*n-1), a(2*n) = 2*n^2.

From Paul Barry, Jun 29 2006: (Start)

G.f.: x*(1+x+2*x^2)/((1+x)^2*(1-x)^3).

a(n) = a(n-1) + 2*a(n-2) - 2*a(n-3) - a(n-4) + a(n-5).

a(n) = n*((2*n+1) - (-1)^n)/4. (End)

a(n) = n * ceiling(n/2). - Wesley Ivan Hurt, Nov 14 2013

E.g.f.: (1/2)*x*( (x+2)*cosh(x) + (x+1)*sinh(x) ). - G. C. Greubel, Mar 14 2024

Sum_{n>=1} 1/a(n) = Pi^2/12 + 2*log(2). - Amiram Eldar, Mar 15 2024

a(n) = A183207(n) - A370980(n) + 1, by Euler's formula. - Scott R. Shannon, Jul 07 2024

A093005:=n->n*ceil(n/2); seq(A093005(n), n=1..100); # Wesley Ivan Hurt, Nov 14 2013

a[n_Integer] := n*Floor[(n + 1)/2] (* Olivier Gérard, Jun 21 2007 *)

Table[n*Ceiling[n/2], {n, 60}] (* or *) LinearRecurrence[{1, 2, -2, -1, 1}, {1, 2, 6, 8, 15}, 60] (* Harvey P. Dale, May 08 2014 *)

(Python)

for n in range(1, 55):

print(n*((n+1)//2), end=", ") # Alex Ratushnyak, Apr 26 2012

(PARI) a(n)=(n+1)\2*n \\ Charles R Greathouse IV, Jun 11 2015

(Magma) [n*(n+(n mod 2))/2: n in [1..70]]; // G. C. Greubel, Mar 14 2024

(SageMath) [n*(n +(n%2))/2 for n in range(1, 71)] # G. C. Greubel, Mar 14 2024

Cf. A000217, A000384, A001105, A006578, A008619 (a(n)/n), A181995.

Cf. A183207, A210977, A210978, A210981, A210982, A370980.

Sequence in context: A228366 A345958 A128913 * A049818 A066189 A278834

Adjacent sequences: A093002 A093003 A093004 * A093006 A093007 A093008

nonn,easy

Amarnath Murthy, Mar 29 2004

Corrected and extended by Joshua Zucker, May 08 2006

New name from Alex Ratushnyak, Apr 26 2012

New name from Wesley Ivan Hurt, Nov 14 2013

approved