Consider the question
Can it be written in closed form?
regarding a recursive function. This question cannot be answered without making the phrase "closed form" more precise, as evidenced by what Wikipedia has to say about closed form:
"It may contain constants, variables, certain well-known operations (e.g., + − × ÷), and functions (e.g., nth root, exponent, logarithm, trigonometric functions, and inverse hyperbolic functions), but usually no limit, differentiation, or integration. The set of operations and functions may vary with author and context."
TODO: Are there any other phrases like this? "Same" immediately comes to mind, but that's much more obviously vague.