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 A229954 The index of Simon's piecewise testability congruence, for words of length 2 over a n-letter alphabet. 0
 3, 16, 152, 2326, 52132, 1602420, 64529264 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Consider an alphabet with n letters, say {a_0, a_1, ... a_{n-1} }. 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 2-equivalent if they have the same subsequences of length up to 2. The n-th term of this sequence is the number of equivalence classes of this equivalence relation, when the size of the alphabet is n. LINKS Prateek Karandikar and Philippe Schnoebelen, On the index of Simon's congruence for piecewise testability arXiv:1310.1278 [cs.FL], 2013-2014. EXAMPLE For n=1, with the alphabet {a_0}, representatives of the three equivalence classes are : empty word, a_0, a_0a_0. CROSSREFS Sequence in context: A217251 A125281 A086371 * A228513 A135753 A191959 Adjacent sequences:  A229951 A229952 A229953 * A229955 A229956 A229957 KEYWORD nonn,more AUTHOR Prateek Karandikar and Philippe Schnoebelen, Oct 09 2013 STATUS approved

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Last modified August 20 16:42 EDT 2018. Contains 313921 sequences. (Running on oeis4.)