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 A002065 a(n+1) = a(n)^2 + a(n) + 1. (Formerly M2961 N1197) 20
 0, 1, 3, 13, 183, 33673, 1133904603, 1285739649838492213, 1653126447166808570252515315100129583, 2732827050322355127169206170438813672515557678636778921646668538491883473 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS a(n) = number of trees of height <= n, generated by unary and binary composition: S = x + (S) + (S,S) = x + (x) + (x,x) + (x,(x)) + ((x),x) + ((x)) + ((x),(x)) + (x,(x,x)) + ((x,x),x) + ((x),(x,x)) + ((x,x),(x)) + ((x,x)) + ((x,x),(x,x)) + ... (x is of height 1); the first difference sequence (beginning with 1), 1 2 10 170 33490 1133870930..., gives the number h(n) of these trees whose height is n, h(n + 1) = h(n) + h(n)*h(n) + 2h(n)*a(n-1), h(1) = 1; as h(n + 1)/h(n) = 1 + a(n) + a(n-1) gives sequence 1, 2, 10 (2*5), 170 (2*5*17), 33490 (2*5*17*197), 1133870930 (2*5*17*197*33877), ... - Claude Lenormand (claude.lenormand(AT)free.fr), Sep 05 2001 This is a divisibility sequence, that is, if n divides m, then a(n) divides a(m). This is a particular case of the result: if p(x) is an integral polynomial then the sequence of n-th iterates p^n(x) (:= p(p^(n-1)(x)) with p^1(x) := p(x)), n = 1,2,..., of p(x) evaluated at x = 0 is a divisibility sequence. In this case p(x) = 1 + x + x^2. - Peter Bala, Mar 28 2018 REFERENCES Steven R. Finch, Mathematical Constants, Cambridge, 2003, pp. 433-434. N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). 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 John Cerkan, 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. Steven R. Finch, Lehmer's Constant [Broken link] Steven R. Finch, Lehmer's Constant [From the Wayback machine] D. H. Lehmer, A cotangent analogue of continued fractions, Duke Math. J., 4 (1935), 323-340. D. H. Lehmer, A cotangent analogue of continued fractions, Duke Math. J., 4 (1935), 323-340. [Annotated scanned copy] H. P. Robinson, Letter to N. J. A. Sloane, Jul 12 1971 Eric Weisstein's World of Mathematics, Lehmer's Constant Eric Weisstein's World of Mathematics, Lehmer Cotangent Expansion Wikipedia Herbrand Structure J. W. Wrench, Jr., Letters to N. J. A. Sloane, Feb 1974 Damiano Zanardini, Computational Logic, Slides, UPM European Master in Computational Logic (EMCL) School of Computer Science Technical University of Madrid. FORMULA a(n) = floor(c^(2^n)) for n > 0, where c = 1.385089248334672909882206535871311526236739234374149506334120193387331772... - Benoit Cloitre, Nov 29 2002 a(n) = (A232806(n) - 1)/2 = (A232806(n-1)^2 + 3)/4. - Peter Bala, Mar 28 2018 MATHEMATICA f[x_] := 1 + x + x^2 ; NestList[f, 1, 7] (* Geoffrey Critzer, May 04 2010 *) PROG (PARI) a(n)=if(n<1, 0, a(n-1)^2+a(n-1)+1) (MAGMA) [n le 1 select 0 else  Self(n-1)^2 + Self(n-1) + 1: n in [1..15]]; // Vincenzo Librandi, Oct 05 2015 (Maxima) a(n) := if n = 0 then 1 else a(n-1)^2+a(n-1)+1 \$ makelist(a(n), n, 0, 8); /* Emanuele Munarini, Mar 23 2017 */ CROSSREFS Cf. A002665, A002794, A002795, A030125, A063573, A232806. Sequence in context: A323134 A081299 A117808 * A302143 A087601 A145503 Adjacent sequences:  A002062 A002063 A002064 * A002066 A002067 A002068 KEYWORD easy,nice,nonn AUTHOR STATUS approved

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Last modified May 25 19:13 EDT 2019. Contains 323576 sequences. (Running on oeis4.)