This site is supported by donations to The OEIS Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A007018 a(n) = a(n-1)^2 + a(n-1), a(0)=1. (Formerly M1713) 31
 1, 2, 6, 42, 1806, 3263442, 10650056950806, 113423713055421844361000442, 12864938683278671740537145998360961546653259485195806 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Number of ordered trees having nodes of outdegree 0,1,2 and such that all leaves are at level n. Example: a(2)=6 because, denoting by I a path of length 2 and by Y a Y-shaped tree with 3 edges, we have I, Y, I*I, I*Y, Y*I, Y*Y, where * denotes identification of the roots. - Emeric Deutsch, Oct 31 2002 a(n) has at least n different prime factors. [Saidak] Subsequence of squarefree numbers (A005117). - Reinhard Zumkeller, Nov 15 2004 [This has been questioned, see MathOverflow link. - Charles R Greathouse IV, Mar 30 2015] For prime factors see A007996. Curtiss shows that if the reciprocal sum of the multiset S = {x_1, x_2, ..., x_n} is 1, then max(S) <= a(n). - Charles R Greathouse IV, Feb 28 2007 The number of reduced ZBDDs for Boolean functions of n variables in which there is no zero sink. (ZBDDs are "zero-suppressed binary decision diagrams.") For example, a(2)=6 because of the 2-variable functions whose truth tables are 1000, 1010, 1011, 1100, 1110, 1111. - Don Knuth, Jun 04 2007 Using the methods of Aho and Sloane, Fibonacci Quarterly 11 (1973), 429-437, it is easy to show that a(n) is the integer just a tiny bit below the real number theta^{2^n}-1/2, where theta =~ 1.597910218 is the exponential of the rapidly convergent series Sum_{n>=0} log(1+1/a_n)/2^{n+1}. For example, theta^32 - 1/2 =~ 3263442.0000000383. - Don Knuth, Jun 04 2007 [Corrected by Darryl K. Nester, Jun 19 2017] a(n) = A139145(2^(n+1) - 1). - Reinhard Zumkeller, Apr 10 2008 a(n+1) = a(n)-th oblong (or promic, pronic, or heteromecic) numbers (A002378). a(n+1) = A002378(a(n)) = A002378(a(n-1)) * (A002378(a(n-1)) + 1). - Jaroslav Krizek, Sep 13 2009 The next term has 209 digits. - Harvey P. Dale, Sep 07 2011 Urquhart shows that a(n) is the minimum size of a tableau refutation of the clauses of the complete binary tree of depth n, see pp. 432-434. - Charles R Greathouse IV, Jan 04 2013 For any positive a(0), the sequence a(n) = a(n-1) * (a(n-1) + 1) gives a constructive proof that there exists integers with at least n distinct prime factors, e.g. a(n). As a corollary, this gives a constructive proof of Euclid's theorem stating that there are an infinity of primes. - Daniel Forgues, Mar 03 2017 Lower bound for A100016 (with equality for the first 5 terms), where a(n)+1 is replaced by nextprime(a(n)). - M. F. Hasler, May 20 2019 REFERENCES R. Honsberger, Mathematical Gems III, M.A.A., 1985, p. 94. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS N. J. A. Sloane, Table of n, a(n) for n = 0..12 A. V. Aho and N. J. A. Sloane, Some doubly exponential sequences, Fib. Quart., 11 (1973), 429-437. D. Adjiashvili, S. Bosio and R. Weismantel, Dynamic Combinatorial Optimization: a complexity and approximability study, 2012. A. Ayyer, A. Schilling, B. Steinberg, N. W. Thiery, Markov chains, R-trivial monoids and representation theory, Int. J. Algebra Comput., 25 (2015), 169-231, arXiv:1401.4250 [math.CO]. Umberto Cerruti, Percorsi tra i numeri (in Italian), page 5. D. R. Curtiss, On Kellogg's Diophantine problem, Amer. Math. Monthly 29 (1922), pp. 380-387. M. Klamkin, ed., Problems in Applied Mathematics: Selections from SIAM Review, SIAM, 1990; see p. 577. Marko R. Riedel, Two-colorings of unordered full binary trees on n levels Matthew Roughan, Surreal Birthdays and Their Arithmetic, arXiv:1810.10373 [math.HO], 2018. Filip Saidak, A New Proof of Euclid's Theorem, Amer. Math. Monthly, Dec 2006 A. Urquhart, The complexity of propositional proofs, Bull. Symbolic Logic, 1 (1995) pp. 425-467, esp. p. 434. Z. Usiskin, Letter to N. J. A. Sloane, Oct. 1991 FORMULA a(n) = A000058(n)-1 = A000058(n-1)^2-A000058(n-1) = 1/(1-sum{j1, a(n-1)+a(n-1)^2, n) \\ Edited by M. F. Hasler, May 20 2019 (Maxima) a:1\$ a[n]:=(a[n-1] + (a[n-1]^2))\$ A007018(n):=a[n]\$ makelist(A007018(n), n, 1, 10); /* Martin Ettl, Nov 08 2012 */ (Haskell) a007018 n = a007018_list !! n a007018_list = iterate a002378 1  -- Reinhard Zumkeller, Dec 18 2013 (MAGMA) [n eq 1 select 1 else Self(n-1)^2 + Self(n-1): n in [1..10]]; // Vincenzo Librandi, May 19 2015 CROSSREFS Cf. A000058, A003687, A004168, A011782, A077125, A117805. Lower bound for A100016. Sequence in context: A054377 A230311 A276416 * A100016 A000610 A023363 Adjacent sequences:  A007015 A007016 A007017 * A007019 A007020 A007021 KEYWORD nonn,nice,easy AUTHOR STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified October 19 04:40 EDT 2019. Contains 328211 sequences. (Running on oeis4.)