Endospores are hardy, defensive structures that enable some bacteria to survive harmful environmental conditions, such as starvation, high temperatures, desiccation (drying out), chemical disinfectants and extremes in pH.
Recursive formula is a formula that is used to determine the next term of a sequence using one or more of the preceding terms.
A recursive definition using just set theory notions is that a (non-empty) binary tree is a triple (L, S, R), where L and R are binary trees or the empty set and S is a singleton set. Some authors allow the binary tree to be the empty set as well.