[27] Use Exercise 6.4.1 to prove Theorem 6.4.1.

Comments. Hint: similar to the proof of Theorem 2.6.1, with the labeled graph G in the part of the overall string, and cover elements (subsets of labeled nodes inducing subgraphs) taking the part of the blocks. Source:

[H.M. Buhrman, M. Li, J.T. Tromp, and P.M.B. Vit´anyi, SIAM J. Comput, 29:2(1999), 590–599]. This is also the source for Exercise 6.4.3.