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 A233130 Number of negative formulas with two connectives (-> and *) and no variables. 1
 1, 0, 2, 6, 38, 210, 1314, 8358, 55118, 370842, 2541626, 17668926, 124321750, 883614498, 6334772562, 45754956054, 332639032734, 2432189656362, 17874332863722, 131957567836206, 978161729926950, 7277592773408562, 54327287358246018, 406792963221032454 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS More precisely, a(n) is the number of negative formulas containing n connectives * or ->, n+1 appearances of symbol "f"=false, and parentheses. Each negative formula is either "f", or is of the form "A->N", where A is a simpler affirmative formula and N is a simpler negative formula. Affirmative formulas are precisely those that are not negative. The total number of formulas, both affirmative and negative, with n connectives * or -> is A151374(n). LINKS Vjekoslav Kovač, Table of n, a(n) for n = 0..1000 V. Čačić and V. Kovač, On the fraction of IL formulas that have normal forms, arXiv:1309.3408 [math.LO], 2013. FORMULA Recurrence: a(0)=1, a(n) = sum_{k=0..n-1} (A151374(k)-a(k)) a(n-k-1). G.f.: (-3 - sqrt(1-8x) + sqrt(10 + 56x + 6*sqrt(1-8x))) / 8x. The ratio a(n)/A151374(n) converges to 1/2 - 3*sqrt(17)/34 as n->infinity. Asymptotics: a(n) ~ (1/2-3*sqrt(17)/34)*8^n/(sqrt(Pi)*n^(3/2)). EXAMPLE a(1)=0 because there are no negative formulas with 1 connective. a(2)=2 because all negative formulas with 2 connectives are: (f->f)->f, (f*f)->f. a(3)=6 because all negative formulas with 3 connectives are: ((f->f)*f)->f, ((f*f)*f)->f, (f->(f->f))->f, (f->(f*f))->f, (f*(f->f))->f, (f*(f*f))->f. MATHEMATICA a = 1; For[n = 1, n <= 23, n++,   a[n] = Sum[(2^k Binomial[2 k, k]/(k + 1) - a[k]) a[n - k - 1], {k,      0, n - 1}]]; Table[a[j], {j, 0, 23}] CROSSREFS Sequence in context: A202737 A186648 A013033 * A267405 A027322 A085447 Adjacent sequences:  A233127 A233128 A233129 * A233131 A233132 A233133 KEYWORD nonn AUTHOR Vjekoslav Kovac, Dec 10 2013 STATUS approved

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Last modified July 31 10:51 EDT 2021. Contains 346373 sequences. (Running on oeis4.)