Nested efficient congruencing and relatives of Vinogradov's mean value theorem

We apply a nested variant of multigrade efficient congruencing to estimate mean values related to that of Vinogradov. We show that when phi j is an element of Z[t] (1 <= j <= k) is a system of polynomials with non-vanishing Wronskian, and s <= k(k+1)/2, then for all complex sequences (an), and for each epsilon>0, one has integral[0,1)k n-ary sumation |n|<= Xane(alpha 1 phi 1(n)+MIDLINE HORIZONTAL ELLIPSIS+alpha k phi k(n))2sd alpha MUCH LESS-THANX epsilon n-ary sumation |n|<= X|an|2s.As a special case of this result, we confirm the main conjecture in Vinogradov's mean value theorem for all exponents k, recovering the recent conclusions of the author (for k=3) and Bourgain, Demeter and Guth (for k > 4). In contrast with the l2-decoupling method of the latter authors, we make no use of multilinear Kakeya estimates, and thus our methods are of sufficient flexibility to be applicable in algebraic number fields, and in function fields. We outline such extensions.