Up, Down, Up
How many permutations (a_1, a_2, ..., a_7) of the numbers 1 through 7 satisfy a_1 < a_2 > a_3 < a_4 > a_5 < a_6 > a_7 ? Give an integer.
Related problems & prerequisites
Worth solving first
- The Frog and Twenty Pads 5.0/10
- No Two Alike in a Row 5.0/10
- The Broken Step 5.0/10
Source: Alternating permutations were counted by Désiré André (1879). Statement written for AxiomIQ.