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Omni-MATH / Count the number of permutations a₁a₂…a₇ of 1234567 with longest decreasing subsequence of length at most two (i.e. there does not exist i<j<k such that a_(i)>a_(j)>a_(k) ).

Problem

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problem

Count the number of permutations a1a2…a7a_{1} a_{2} \ldots a_{7} of 1234567 with longest decreasing subsequence of length at most two (i.e. there does not exist i<j<ki<j<k such that ai>aj>aka_{i}>a_{j}>a_{k} ).
Plain-text mathematical notation (without MathML)
Count the number of permutations a₁a₂…a₇ of 1234567 with longest decreasing subsequence of length at most two (i.e. there does not exist i<j<k such that a_(i)>a_(j)>a_(k) ).
Original LaTeX notation
Count the number of permutations $a_{1} a_{2} \ldots a_{7}$ of 1234567 with longest decreasing subsequence of length at most two (i.e. there does not exist $i<j<k$ such that $a_{i}>a_{j}>a_{k}$ ).

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Source and history

task_id
7d5c3538-1ecc-5b83-8196-79ddde34ad88
task_key
test--7d5c3538-1ecc-5b83-8196-79ddde34ad88
task_revision_id
1
config / split
/ test

Official source

initial import