Eli is a student who loves chemistry and recently joined a chemistry lab. There are N test tubes in a row, numbered 1,2,…,N from the left. Eli chooses some of the tubes and mixes them into one mixture. A mixture must satisfy both rules below.
- Never choose two tubes whose numbers are adjacent.
- No more tubes can be added without breaking rule 1. That is, every unchosen tube is adjacent to at least one chosen tube.
A choice of only one tube does not count as a mixture, so singletons are not counted.
For example, for N=5 the valid mixtures are {1,3,5}, {2,4}, {2,5}, and {1,4}, which is 4 in total. {1,3} does not count because tube 5 can still be added without breaking rule 1, so it violates rule 2.
For each given number N of tubes, compute the number of different mixtures satisfying the rules.