cp's OEIS Frontend

A369886 a(n) is the least positive integer that can be expressed as +- 1 +- 2 +- 3 +- ... +- n in exactly n ways, or -1 if no such integer exists.

%I A369886 #5 Feb 04 2024 18:31:38
%S A369886 1,-1,-1,-1,-1,-1,6,16,25,35,-1,56,-1,-1,96,-1,-1,145,-1,-1,-1,225,-1,
%T A369886 -1,-1,-1,348,-1,-1,-1,-1,496,-1,-1,-1,-1,-1,707,-1,-1,-1,-1,-1,-1,-1,
%U A369886 1045,-1,-1,-1,-1,-1,-1,-1,1447,-1
%N A369886 a(n) is the least positive integer that can be expressed as +- 1 +- 2 +- 3 +- ... +- n in exactly n ways, or -1 if no such integer exists.
%e A369886 a(7) = 6: 6 = + 1 + 2 + 3 - 4 + 5 + 6 - 7
%e A369886             = - 1 + 2 - 3 + 4 + 5 + 6 - 7
%e A369886             = + 1 + 2 + 3 + 4 - 5 - 6 + 7
%e A369886             = - 1 + 2 + 3 - 4 + 5 - 6 + 7
%e A369886             = + 1 - 2 - 3 + 4 + 5 - 6 + 7
%e A369886             = + 1 - 2 + 3 - 4 - 5 + 6 + 7
%e A369886             = - 1 - 2 - 3 + 4 - 5 + 6 + 7.
%Y A369886 Cf. A038547, A063890.
%K A369886 sign
%O A369886 1,7
%A A369886 _Ilya Gutkovskiy_, Feb 04 2024