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 A137744 Number of different strings of length n obtained from "abcd" by iteratively duplicating any substring. 12
 0, 0, 0, 0, 1, 4, 13, 40, 119, 348, 1014, 2966, 8726, 25820, 76823, 229814, 691186, 2089850, 6351448, 19398726, 59525641, 183462778 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,6 COMMENTS See A137743 for more comments. LINKS EXAMPLE a(4) = # { abcd }, a(5) = # { aabcd, abbcd, abccd, abcdd }, a(6) = # { aaabcd, aabbcd, aabccd, aabcdd, ababcd, abbbcd, abbccd, abbcdd, abcbcd, abcccd, abccdd, abcdcd, abcddd } PROG (PARI) A135473(12, 4) (Python) def process(s, n, catalog, cache): ....l=len(s) ....if l==n: ........catalog.add(s) ........return ....if s in cache: ........return ....cache.add(s) ....for x in range(l): ........for y in range(x+1, min(x+n-l, l)+1): ............process(s[:y]+s[x:], n, catalog, cache) def A137744(n): ....catalog=set() ....cache=set() ....process("abcd", n, catalog, cache) ....return len(catalog) # Bert Dobbelaere, Nov 01 2018 CROSSREFS Cf. A137740-A137743, A135473, A137745-A137748. Sequence in context: A272581 A191132 A119915 * A027130 A027121 A238846 Adjacent sequences:  A137741 A137742 A137743 * A137745 A137746 A137747 KEYWORD more,nonn AUTHOR M. F. Hasler, Feb 10 2008 EXTENSIONS a(13)-a(19) from Lars Blomberg, Jan 12 2013 a(20)-a(21) from Bert Dobbelaere, Nov 01 2018 STATUS approved

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Last modified December 19 02:33 EST 2018. Contains 318245 sequences. (Running on oeis4.)