Number of unique nonempty strings with either 0 or 1 copies of a letter i_1, either 0 or 2 copies of a letter i_2, ..., either 0 or n copies of a letter i_n, where i_1, ..., i_n are all distinct letters.

A389772

Number of unique nonempty strings with either 0 or 1 copies of a letter i_1, either 0 or 2 copies of a letter i_2, ..., either 0 or n copies of a letter i_n, where i_1, ..., i_n are all distinct letters.