1.2.9. (-) What is the minimum number of trails needed to decompose the Petersen graph? Is there a decomposition into this many trails using only paths?

Write 20 as the direct sum of two of its nontrivial subgroups.

2. Suppose n ≥ 1 is an integer. Consider an (n + 1) x (n + 1) grid of integer points; i.e. points of the form (a, b) where 0 ≤ a,b ≤n. A Binomial Path with 2n steps is a path from the point (0, 0) to (n, n) formed by moving either 'right' (i.e. from (a, b) to (a +1, b)) or ‘up' (i.e. from (a, b) to (a, b+1)). (a) Draw all distinct Binomial Paths with 2n steps when n = = 2. (b) Write down a correspondence that relates the Binomial Paths with 2n steps to strings of length 2n consisting of exactly n 1s and n Os. More precisely, let B₁, be the set of Binomial Paths with 2n steps, and let Sn be the set of strings of length 2n consisting of exactly n 1s and n Os. Construct a bijection f: Bn Sn. (You don't have to prove that it is a bijection.)

5. Can somcone cross all the bridges shown in this map cxactly once and return to the starting point?