

A229954


The index of Simon's piecewise testability congruence, for words of length 2 over an nletter alphabet.


0




OFFSET

1,1


COMMENTS

Consider an alphabet with n letters, say {a_0, a_1, ... a_{n1} }. For two words v and w over this alphabet, we say v embeds in w, or v is a subsequence of w, if v can be obtained from w by erasing some (occurrences of) letters.
Define two words to be 2equivalent if they have the same subsequences of length up to 2. The nth term of this sequence is the number of equivalence classes of this equivalence relation, when the size of the alphabet is n.


LINKS



EXAMPLE

For n=1, with the alphabet {a_0}, representatives of the three equivalence classes are: empty word, a_0, a_0a_0.


CROSSREFS



KEYWORD

nonn,more


AUTHOR



STATUS

approved



