Post 1946 PCP: given (a_i, b_i) pairs, find sequence i_1...i_n with a_{i_1}...a_{i_n} = b_{i_1}...b_{i_n}; PCP undecidable; reduction-base for many-one undecidability proofs.
Post 1946 PCP: given (a_i, b_i) pairs, find sequence i_1...i_n with a_{i_1}...a_{i_n} = b_{i_1}...b_{i_n}; PCP undecidable; reduction-base for many-one undecidability proofs.