login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A173395 a(n) = (A002260(n) + 1) * (A004736(n) + 1). 3

%I #25 Mar 12 2021 22:24:46

%S 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,

%T 24,21,16,18,24,28,30,30,28,24,18,20,27,32,35,36,35,32,27,20,22,30,36,

%U 40,42,42,40,36,30,22,24,33,40,45,48,49,48,45,40,33,24,26

%N a(n) = (A002260(n) + 1) * (A004736(n) + 1).

%C Every number of this sequence is composite, and every composite number appears in this sequence.

%C 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

%H Michael De Vlieger, <a href="/A173395/b173395.txt">Table of n, a(n) for n = 1..11325</a> (rows 1 <= n <= 150).

%H Michael Somos, <a href="https://oeis.org/A073189/a073189.txt">Sequences used for indexing triangular or square arrays</a>.

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

%e 4;

%e 6,6;

%e 8,9,8;

%e 10,12,12,10;

%e 12,15,16,15,12;

%e From _Luc Rousseau_, Oct 15 2017: (Start)

%e Viewed as a square array,

%e 4 6 8 10 12 ...

%e 6 9 12 15 18 ...

%e 8 12 16 20 24 ...

%e 10 15 20 25 30 ...

%e 12 18 24 30 36 ...

%e ...

%e = the multiplication table with headers starting at 2.

%e (End)

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

%o (Other) a(n) = (1/2 (2 n+Round[sqrt(2 n)]-Round[sqrt(2 n)]^2)+1) (1/2 (2-2 n+Round[sqrt(2 n)]+Round[sqrt(2 n)]^2)+1)

%o (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

%o (PARI) a(n)=my(s=round(sqrt(n*=2)));(n-s-s^2-4)*(n+s-s^2+2)/4 \\ _Charles R Greathouse IV_, Jun 19 2013

%Y Cf. A002260, A004736.

%K nonn,easy,tabl

%O 1,1

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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 28 08:22 EDT 2024. Contains 371236 sequences. (Running on oeis4.)