Av(1243, 1342, 2143, 2413)
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Generating Function
\(\displaystyle \frac{-2 x +1-\sqrt{8 x^{4}-20 x^{3}+20 x^{2}-8 x +1}}{2 x \left(x -1\right)^{2}}\)
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Counting Sequence
1, 1, 2, 6, 20, 69, 244, 882, 3250, 12174, 46244, 177769, 690394, 2705023, 10680150, ...
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Implicit Equation for the Generating Function
\(\displaystyle x \left(x -1\right)^{2} F \left(x \right)^{2}+\left(2 x -1\right) F \! \left(x \right)-2 x +1 = 0\)
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Recurrence
\(\displaystyle a \! \left(0\right) = 1\)
\(\displaystyle a \! \left(1\right) = 1\)
\(\displaystyle a \! \left(2\right) = 2\)
\(\displaystyle a \! \left(3\right) = 6\)
\(\displaystyle a \! \left(4\right) = 20\)
\(\displaystyle a \! \left(n +5\right) = \frac{8 \left(1+n \right) a \! \left(n \right)}{n +6}-\frac{2 \left(14 n +25\right) a \! \left(1+n \right)}{n +6}+\frac{10 \left(4 n +11\right) a \! \left(n +2\right)}{n +6}-\frac{4 \left(7 n +26\right) a \! \left(n +3\right)}{n +6}+\frac{\left(43+9 n \right) a \! \left(n +4\right)}{n +6}, \quad n \geq 5\)
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This specification was found using the strategy pack "Point Placements" and has 13 rules.

Found on July 23, 2021.

Finding the specification took 2 seconds.

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\(\begin{align*} F_{0}\! \left(x \right) &= F_{1}\! \left(x \right)+F_{2}\! \left(x \right)\\ F_{1}\! \left(x \right) &= 1\\ F_{2}\! \left(x \right) &= F_{3}\! \left(x \right)\\ F_{3}\! \left(x \right) &= F_{12}\! \left(x \right) F_{4}\! \left(x \right)\\ F_{4}\! \left(x \right) &= F_{0}\! \left(x \right)+F_{5}\! \left(x \right)\\ F_{5}\! \left(x \right) &= F_{6}\! \left(x \right)\\ F_{6}\! \left(x \right) &= F_{0}\! \left(x \right) F_{12}\! \left(x \right) F_{7}\! \left(x \right)\\ F_{7}\! \left(x \right) &= F_{4}\! \left(x \right)+F_{8}\! \left(x \right)\\ F_{8}\! \left(x \right) &= F_{0}\! \left(x \right) F_{9}\! \left(x \right)\\ F_{9}\! \left(x \right) &= F_{10}\! \left(x \right)\\ F_{10}\! \left(x \right) &= F_{11}\! \left(x \right) F_{12}\! \left(x \right)\\ F_{11}\! \left(x \right) &= 2 F_{10}\! \left(x \right)+F_{1}\! \left(x \right)\\ F_{12}\! \left(x \right) &= x\\ \end{align*}\)