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

1,7

Links

Table of n, a(n) for n=1..55.

Example

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.

Crossrefs

Cf. A038547, A063890.

Sequence in context: A114222 A348229 A275641 * A171440 A031220 A011536

Adjacent sequences: A369883 A369884 A369885 * A369887 A369888 A369889

Keyword

sign

Author

Ilya Gutkovskiy, Feb 04 2024

Status

approved