A062855 - OEIS

%I #5 Oct 14 2023 00:04:32

%S 0,1,2,4,4,7,6,10,11,13,10,17,12,19,20,23,16,26,18,29,29,31,22,37,33,

%T 37,38,42,28,47,30,48,47,49,49,58,36,55,56,63,40,66,42,67,68,67,46,78,

%U 66,77,74,80,52,85,78,89,83

%N First differences of A062851.

%F a(n) = 2*n-1-A062854(n). - _Chai Wah Wu_, Oct 13 2023

%e a(4)=4 because there are 7 integer non-products for the 4 X 4 multiplication table (5 7 10 11 13 14 15), which is 4 more than the 3 non-products for the 3 X 3 multiplication table (5 7 8).

%o (Python)

%o from itertools import takewhile

%o from sympy import divisors

%o def A062855(n): return (n<<1)-1-sum(1 for i in range(1,n+1) if all(d<=i for d in takewhile(lambda d:d<n,divisors(n*i)))) # _Chai Wah Wu_, Oct 13 2023

%Y Cf. A027424, A062851, A062854, A062856, A062857, A062858, A062859.

%K nonn

%O 1,3

%A Ron A. Lalonde (ronronronlalonde(AT)hotmail.com), Jun 25 2001