A027427 - OEIS

%I #25 Oct 13 2023 18:52:54

%S 0,1,2,4,7,11,14,20,25,32,37,47,52,64,71,79,88,104,112,130,140,151,

%T 162,184,193,211,224,240,253,281,292,322,338,355,372,391,404,440,459,

%U 479,494,534,550,592,612,632,655,701,718,753,775,801,824,876

%N Number of distinct products ij with 0 <= i < j <= n.

%H T. D. Noe, <a href="/A027427/b027427.txt">Table of n, a(n) for n = 0..1000</a>

%F a(n) = A027428(n)+1. - _T. D. Noe_, Jan 16 2007

%p A027427 := proc(n)

%p     local L, i, j ;

%p     L := {};

%p     for i from 0 to n do

%p         for j from i+1 to n do

%p             L := L union {i*j};

%p         end do:

%p     end do:

%p     nops(L);

%p end proc:  # _R. J. Mathar_, Jun 09 2016

%t a[n_] := Table[i*j, {i, 0, n}, {j, i+1, n}] // Flatten // Union // Length;

%t Table[a[n], {n, 0, 60}] (* _Jean-François Alcover_, Feb 03 2018 *)

%o (Haskell)

%o import Data.List (nub)

%o a027427 n = length $ nub [i*j | j <- [1..n], i <- [0..j-1]]

%o -- _Reinhard Zumkeller_, Jan 01 2012

%o (Python)

%o def A027427(n): return 1+len({i*j for i in range(1,n+1) for j in range(1,i)}) if n else 0 # _Chai Wah Wu_, Oct 13 2023

%Y Cf. A027430, etc.

%Y Cf. A027384, A027428, A027429.

%K nonn

%O 0,3

%A _N. J. A. Sloane_.