A026439 - OEIS

A026439

a(n) = least positive integer > a(n-1) and not a(i)*a(j)-1 for 1 <= i <= j < n, n >= 2, with initial terms 2,3.

%I #18 Feb 06 2025 13:51:24

%S 2,3,4,6,9,10,12,13,14,16,18,20,21,22,24,28,30,32,33,34,36,37,40,42,

%T 44,45,46,48,49,50,52,54,56,57,58,60,61,64,66,68,69,70,72,74,75,76,78,

%U 81,82,84,85,86,88,90,92,93,94,96,100,102

%N a(n) = least positive integer > a(n-1) and not a(i)*a(j)-1 for 1 <= i <= j < n, n >= 2, with initial terms 2,3.

%H Ivan Neretin, <a href="/A026439/b026439.txt">Table of n, a(n) for n = 1..1000</a>

%t a = {2, 3}; used = Flatten@Outer[Times, a, a] - 1; Do[k = a[[-1]] + 1; While[MemberQ[used, k], k++]; AppendTo[a, k]; used = Union[used, k*a - 1], {n, 3, 60}]; a (* _Ivan Neretin_, Feb 15 2018 *)

%Y Cf. A026437 and references therein.

%K nonn

%O 1,1

%A _Clark Kimberling_

%E Name clarified by _Robert C. Lyons_, Feb 06 2025