Av(123, 231, 2143)
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Generating Function
\(\displaystyle \frac{x^{3}+x^{2}-x +1}{\left(x -1\right)^{2}}\)
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Counting Sequence
1, 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, ...
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Implicit Equation for the Generating Function
\(\displaystyle -\left(x -1\right)^{2} F \! \left(x \right)+x^{3}+x^{2}-x +1 = 0\)
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Recurrence
\(\displaystyle a \! \left(0\right) = 1\)
\(\displaystyle a \! \left(1\right) = 1\)
\(\displaystyle a \! \left(2\right) = 2\)
\(\displaystyle a \! \left(3\right) = 4\)
\(\displaystyle a \! \left(n \right) = -2+2 n, \quad n \geq 4\)
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Explicit Closed Form
\(\displaystyle \left\{\begin{array}{cc}1 & n =0\text{ or } n =1 \\ -2+2 n & \text{otherwise} \end{array}\right.\)
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This specification was found using the strategy pack "Point Placements" and has 15 rules.

Found on January 17, 2022.

Finding the specification took 1 seconds.

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\(\begin{align*} F_{0}\! \left(x \right) &= F_{1}\! \left(x \right)+F_{2}\! \left(x \right)\\ F_{1}\! \left(x \right) &= 1\\ F_{2}\! \left(x \right) &= F_{3}\! \left(x \right)\\ F_{3}\! \left(x \right) &= F_{4}\! \left(x \right) F_{5}\! \left(x \right)\\ F_{4}\! \left(x \right) &= x\\ F_{5}\! \left(x \right) &= F_{6}\! \left(x \right)+F_{9}\! \left(x \right)\\ F_{6}\! \left(x \right) &= F_{1}\! \left(x \right)+F_{7}\! \left(x \right)\\ F_{7}\! \left(x \right) &= F_{8}\! \left(x \right)\\ F_{8}\! \left(x \right) &= F_{4}\! \left(x \right) F_{6}\! \left(x \right)\\ F_{9}\! \left(x \right) &= F_{10}\! \left(x \right)+F_{2}\! \left(x \right)\\ F_{10}\! \left(x \right) &= F_{11}\! \left(x \right)\\ F_{11}\! \left(x \right) &= F_{12}\! \left(x \right) F_{4}\! \left(x \right)\\ F_{12}\! \left(x \right) &= F_{13}\! \left(x \right)+F_{14}\! \left(x \right)\\ F_{13}\! \left(x \right) &= F_{4}\! \left(x \right)\\ F_{14}\! \left(x \right) &= F_{11}\! \left(x \right)\\ \end{align*}\)