Let λ1, λ2, λ3, λ4, λ5 be nonzero real numbers, neither all positive nor all negative and λ1/λ2 be an irrational number. Let V be a well-spaced sequence and δ > 0. For arbitrary ɛ > 0, we show that the quantity of v ∈ V with v ≤ N making the inequality |λ1p12 + λ2p22 + λ3p34 + λ4p44 + λ5p54 − v| < v−δ unsolvable in primes p1,p2,p3,p4,p5 does not exceed O(N6/7 + 2δ + ε).
Fu et al. (2026) studied this question.