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.
In the last case above, we were able to come up with a regular formula (a closed form expression) for the sequence; this is often not possible (or at least not reasonable) for recursive sequences, which is why you need to keep them in mind as a difference class of sequences.