Enumerative combinatorics
A combinatorial interpretation of the recurrence fn+1= 6fn - fn-1
Discrete Mathematics
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In the existing literature there are many combinatorial interpretations for the sequence (ai)i≥1 = 1,3,7,17,41,... (M2665 in The Encyclopedia of Integer Sequences, Academic Press, New York, 1995), and at least one for the subsequence of its odd-indexed terms, i.e. 1,7,41,239,..., using the area under elevated Schröder paths. We provide a combinatorial interpretation for the subsequence given by the remaining terms 3,17,99,..., also in this case by using the area under Schröder paths.