A general case:
we have totally n letters, among them
n_1 times A, n_2 times B, n_3 times C,
n_4 times D etc.
(n_1 ,n_2 ,n_3 ,n_(4,) ...≥2, n>n_1 +n_2 +n_3 +n_4 +....)
how many different words can be
formed using these n letters such that
same letters are not next to each
other.
see also Q107451.
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