A026440 - OEIS

A026440

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 1,4.

%I #18 Feb 06 2025 13:51:20

%S 1,4,5,6,7,8,9,10,11,12,13,14,16,17,18,20,21,22,25,26,28,30,32,33,36,

%T 37,38,40,42,45,46,50,52,56,57,58,60,61,66,68,70,72,73,74,75,78,81,82,

%U 85,86,88,91,92,93,94,96,100,102,105,106

%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 1,4.

%H Ivan Neretin, <a href="/A026440/b026440.txt">Table of n, a(n) for n = 1..1000</a>

%t a = {1, 4}; 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,changed

%O 1,2

%A _Clark Kimberling_

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