a) Prove that {∨, ∧, ¬} is functional complete
b) Prove that the singleton sets {|} and {↓} where
$$ \begin{array}{cc|ccccc|c} x & y & x \mid y & & x & y & x \downarrow y \\ \hline 0 & 0 & 1 & & 0 & 0 & 1 \\ 0 & 1 & 1 & & 0 & 1 & 0 \\ 1 & 0 & 1 & & 1 & 0 & 0 \\ 1 & 1 & 0 & & 1 & 1 & 0 \end{array} $$
are functional complete. The connective | is called Sheffer stroke and the connective ↓ is called Pierce arrow.
c) Prove that the sets {∨, ∧}, {→, ∨}, {→, ∧} are not adequate sets of connectives.