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.
1, -1, -1, -1, -1, -1, 6, 16, 25, 35, -1, 56, -1, -1, 96, -1, -1, 145, -1, -1, -1, 225, -1, -1, -1, -1, 348, -1, -1, -1, -1, 496, -1, -1, -1, -1, -1, 707, -1, -1, -1, -1, -1, -1, -1, 1045, -1, -1, -1, -1, -1, -1, -1, 1447, -1
Offset: 1
Keywords
Examples
a(7) = 6: 6 = + 1 + 2 + 3 - 4 + 5 + 6 - 7
= - 1 + 2 - 3 + 4 + 5 + 6 - 7
= + 1 + 2 + 3 + 4 - 5 - 6 + 7
= - 1 + 2 + 3 - 4 + 5 - 6 + 7
= + 1 - 2 - 3 + 4 + 5 - 6 + 7
= + 1 - 2 + 3 - 4 - 5 + 6 + 7
= - 1 - 2 - 3 + 4 - 5 + 6 + 7.