1, single propositional constants or variables p, q, r, ... and 0, 1 are compound formulas.

2. if a is compound, (~ a) is also compound. (Note: the symbol ~ is regarded as a negative number here)

3. If a and b are compound formulas, then (A ∧ B), (A∨B), (A→B) and (A∧B) are also compound formulas. (Note: The symbols here are regarded as equivalent)

4. Only the symbol string composed of 1, 2, 3 is a formula if it is applied a limited number of times.

In addition, in propositional logic, combinatorial formulas are also called propositional formulas, or formulas for short.

In addition: For convenience, the brackets specifying (~ a) and (A∧B) can be omitted.

-The above quotation comes from the fifth page of Discrete Mathematics (fifth edition) edited by Geng Suyun of Tsinghua University Publishing House.

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According to the definition, p→(r→q) is a compound, and (p→(r→q)) is also a compound. If they are not combined formulas, it can only show that pqr is not a combined formula.