Number of functions from [1,2,...,n] to [1,2,...,n] such that the image contains exactly two elements.

A068605

Number of functions from [1,2,...,n] to [1,2,...,n] such that the image contains exactly two elements.

Terms

    a(0) =2a(1) =18a(2) =84a(3) =300a(4) =930a(5) =2646a(6) =7112a(7) =18360a(8) =45990a(9) =112530a(10) =270204a(11) =638820a(12) =1490762a(13) =3440430a(14) =7864080a(15) =17825520a(16) =40107726a(17) =89652906a(18) =199229060a(19) =440401500a(20) =968883762a(21) =2122317318a(22) =4630511064a(23) =10066329000a(24) =21810380150a(25) =47110421826a(26) =101468601612a(27) =217969589460a(28) =467077692570

External references