



4, 6, 6, 8, 9, 8, 10, 12, 12, 10, 12, 15, 16, 15, 12, 14, 18, 20, 20, 18, 14, 16, 21, 24, 25, 24, 21, 16, 18, 24, 28, 30, 30, 28, 24, 18, 20, 27, 32, 35, 36, 35, 32, 27, 20, 22, 30, 36, 40, 42, 42, 40, 36, 30, 22, 24, 33, 40, 45, 48, 49, 48, 45, 40, 33, 24, 26
(list;
table;
graph;
refs;
listen;
history;
text;
internal format)



OFFSET

1,1


COMMENTS

Every number of this sequence is composite, and every composite number appears in this sequence.
Viewed as a square array this sequence is the multiplication table with headers starting at 2: A002260 and A004736 being indexing functions for square arrays, a(n)=T(i,j) with i=A002260(n) and j=A004736(n), T(i,j)=(i+1)(j+1).  Luc Rousseau, Oct 15 2017


LINKS



FORMULA

a(n) = ((2 n + round(sqrt(2n))  round(sqrt(2n))^2)/2 + 1)((2  2n + round(sqrt(2n)) + round(sqrt(2n))^2)/2 + 1).


EXAMPLE

4;
6,6;
8,9,8;
10,12,12,10;
12,15,16,15,12;
Viewed as a square array,
4 6 8 10 12 ...
6 9 12 15 18 ...
8 12 16 20 24 ...
10 15 20 25 30 ...
12 18 24 30 36 ...
...
= the multiplication table with headers starting at 2.
(End)


MATHEMATICA

Map[Times @@ # & /@ Transpose@{#, Reverse@ #} &, Array[Range, 12] + 1] // Flatten (* Michael De Vlieger, Oct 16 2017 *)


PROG

(Other) a(n) = (1/2 (2 n+Round[sqrt(2 n)]Round[sqrt(2 n)]^2)+1) (1/2 (22 n+Round[sqrt(2 n)]+Round[sqrt(2 n)]^2)+1)
(PARI) a(n) = ((2*n + round(sqrt(2*n))  round(sqrt(2*n))^2)/2 + 1)*((2  2*n + round(sqrt(2*n)) + round(sqrt(2*n))^2)/2 + 1) \\ Michel Marcus, Jun 19 2013


CROSSREFS



KEYWORD



AUTHOR

Fabio Civolani (civox(AT)tiscali.it), Feb 17 2010


STATUS

approved



