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 A004019 a(0) = 0; for n > 0, a(n) = (a(n-1) + 1)^2. (Formerly M3611) 11
 0, 1, 4, 25, 676, 458329, 210066388900, 44127887745906175987801, 1947270476915296449559703445493848930452791204, 3791862310265926082868235028027893277370233152247388584761734150717768254410341175325352025 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Take the standard rooted binary tree of depth n, with 2^(n+1) - 1 labeled nodes. Here is a picture of the tree of depth 3:              R             / \            /   \           /     \          /       \         /         \        o           o       / \         / \      /   \       /   \     o     o     o     o    / \   / \   / \   / \   o   o o   o o   o o   o Let the number of rooted subtrees be s(n). For example, for n = 1 the s(2) = 4 subtrees are:   R   R   R      R      /     \    / \     o       o  o   o Then s(n+1) = 1 + 2*s(n) + s(n)^2 = (1+s(n))^2 and so s(n) = a(n+1). REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). Mordechai Ben-Ari, Mathematical Logic for Computer Science, Third edition, 173-203. LINKS Reinhard Zumkeller, Table of n, a(n) for n = 0..11 (shortened by N. J. A. Sloane, Jan 13 2019) A. V. Aho and N. J. A. Sloane, Some doubly exponential sequences, Fib. Quart., 11 (1973), 429-437. R. P. Stanley, Letter to N. J. A. Sloane, c. 1991 Elmar Teufl and Stephan Wagner, Enumeration problems for classes of self-similar graphs, Journal of Combinatorial Theory, Series A, Volume 114, Issue 7, October 2007, Pages 1254-1277. Wikipedia, Herbrand Structure Damiano Zanardini, Computational Logic, UPM European Master in Computational Logic (EMCL) School of Computer Science Technical University of Madrid. FORMULA a(n) = A003095(n)^2 = A003095(n+1) - 1 = A056207(n+1) + 1. It follows from Aho and Sloane that there is a constant c such that a(n) is the nearest integer to c^(2^n). In fact a(n+1) = nearest integer to b^(2^n) - 1 where b = 2.25851845058946539883779624006373187243427469718511465966.... - Henry Bottomley, Aug 30 2005 MATHEMATICA Table[Nest[(1 + #)^2 &, 0, n], {n, 0, 12}] (* Vladimir Joseph Stephan Orlovsky, Jul 20 2011 *) NestList[(#+1)^2&, 0, 10] (* Harvey P. Dale, Oct 08 2011 *) PROG (Haskell) a004019 n = a004019_list !! n a004019_list = iterate (a000290 . (+ 1)) 0 -- Reinhard Zumkeller, Feb 01 2013 (MAGMA) [n le 1 select 0 else  (Self(n-1)+1)^2: n in [1..15]]; // Vincenzo Librandi, Oct 05 2015 (PARI) a(n) = if(n==0, 0, (a(n-1) + 1)^2); vector(20, n, a(n-1)) \\ Altug Alkan, Oct 06 2015 CROSSREFS Cf. A001699, A056207. Cf. A000290. Sequence in context: A167041 A123129 A075577 * A302092 A277110 A072882 Adjacent sequences:  A004016 A004017 A004018 * A004020 A004021 A004022 KEYWORD nonn,easy,nice AUTHOR EXTENSIONS One more term from Henry Bottomley, Jul 24 2000 Additional comments from Max Alekseyev, Aug 30 2005 STATUS approved

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Last modified August 24 14:26 EDT 2019. Contains 326285 sequences. (Running on oeis4.)