The Pfaffian transform.
The problem whether each element of a sequence satisfying a fourth order linear recurrence with integer coefficients is nonnegative, referred to as the Positivity Problem for fourth order linear recurrence sequence, is shown to be decidable.
Let be fixed positive integers, and let be any set of positive integers. Let denote the set of all integers representable as a sum of no more than elements of , and let denote the largest integer such that . Let , where the maximum is taken over all sets with elements. We determine when the elements of are in geometric progression. In particular, this results in the evaluation of and yields surprisingly sharp lower bounds for , particularly for .