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 A138156 Sum of the path lengths of all binary trees with n edges. 4
 0, 2, 14, 74, 352, 1588, 6946, 29786, 126008, 527900, 2195580, 9080772, 37392864, 153434536, 627778954, 2562441466, 10438340104, 42449348236, 172376641924, 699100282156, 2832205421824, 11462854280536, 46354571222164 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS a(n) = 2*A006419(n). If (2*n+3) prime, then A138156(n) mod (2*n+3) == 0. - Alzhekeyev Ascar M, Jul 19 2011 REFERENCES D. E. Knuth, The Art of Computer Programming. Addison-Wesley, Reading, MA, 1997, Vol. 1, p. 405 (exercise 5) and p. 595 (solution). LINKS FORMULA a(n) = 4^(n+1) - (3*n+4) * C(2*n+2,n+1)/(n+2). G.f.: 1/(z*(1-4*z)) - ((1-z)/sqrt(1-4*z)-1)/z^2. EXAMPLE a(1) = 2 because the trees with one edge are (i) root with a left child and (ii) root with a right child, each having path length 1. MAPLE a:= n-> 4^(n+1)-(3*n+4)*binomial(2*n+2, n+1)/(n+2): seq(a(n), n=0..22); MATHEMATICA Table[4^(n+1)-(3n+4) Binomial[2n+2, n+1]/(n+2), {n, 0, 30}] (* Harvey P. Dale, Dec 14 2014 *) CROSSREFS Cf. A095830, A138157, A006419. Sequence in context: A189305 A043011 A290698 * A280392 A192809 A198762 Adjacent sequences:  A138153 A138154 A138155 * A138157 A138158 A138159 KEYWORD nonn AUTHOR Emeric Deutsch, Mar 20 2008 STATUS approved

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Last modified October 20 08:05 EDT 2019. Contains 328252 sequences. (Running on oeis4.)