Linked Fates: How Small of an Ambiguity Increase Can Make the Difference Between Equaling and Separating from P?
Ambiguity-bounded versions of $\mathrm{NP}$, denoted $\mathrm{UP}_{\leq f(n)}$, bound by $f(n)$ the number of accepting paths the nondeterministic polynomial-time Turing machine can have on inputs of length $n$. Such classes range from Valiant's completely unambiguous ($f(n)=1$) class $\mathrm{UP}$ to $\mathrm{NP}$ itself, where there is no bound or, equivalently, there is the toothless exponential bound ($f(n) = 2^{n^{O(1)}}$). This paper seeks to understand which of these classes stand and fall together as to whether they equal deterministic polynomial time. Informally put, what ranges of ambiguities have linked fates? That is, for which pairs of nondecreasing functions, $(f_1 ,f_2)$, satisfying $(\forall n)[f_1(n) \leq f_2(n)]$, does it hold that $\mathrm{P} = \mathrm{UP}_{\leq f_1(n)} \implies \mathrm{P} = \mathrm{UP}_{\leq f_2(n)}$. More particularly, for which pairs does that hold robustly, i.e., it holds in the real world and every relativized world? And for which pairs does that implication fail to hold robustly, i.e., there is an oracle $A$ such that $\mathrm{P}^A = \mathrm{UP}_{\leq f_1(n)}^A \subsetneq \mathrm{UP}_{\leq f_2(n)}^A$? The only previously known positive result is Watanabe's 1988 result that $ \mathrm{P} = \mathrm{UP}_{\leq 1} \implies (\forall k \geq 1)[\mathrm{P} = \mathrm{UP}_{\leq k}]$, which even holds robustly. His result, though lovely, applies only to constant-bounded ambiguities. As our positive result, we present a new class of cases (Theorem 3.8) that apply (and even robustly apply) at greater ambiguity levels. To give our class of cases, we leverage two approaches: a novel path-poisoning approach that works even on superconstant ambiguities (Theorem 3.5) and a new application of the power of padding (Theorems 3.3/3.4). As negative results, we show that for essentially all other cases, no linkage holds robustly.