The second-order expressions of Boolean functions can have either sum-of-product or product-of-sum forms. For a Boolean function specified in the irredundant sum-of-product form as the disjunction of a number of prime implicants or p terms, groups of these p terms can sometimes be more economically realized in the minimal product-of-sum forms than in the sum-of-product forms. To know whether a group of p terms in the irredundant sum-of-product form of the function has a more economic realization in the product-of-sum form, the concept of coincidence between the p terms of the function is introduced in the paper and a number of interesting properties of the function in relation to coincidence are established. The coincidence between a pair of p terms in a function is defined as the number of literals occurring as mutually common in their algebraic representations. It is next shown that the study of the properties of Boolean functions in relation to coincidence also aids in readily obtaining the economic third-order expressions of general Boolean functions.
