We study bounded ancient solutions of the Navier–Stokes equations. These are solutions with bounded velocity defined in$R$^{$n$}× (−1, 0). In two space dimensions we prove that such solutions are either constant or of the form$u$($x$,$t$) =$b$($t$), depending on the exact definition of admissible solutions. The general 3-dimensional problem seems to be out of reach of existing techniques, but partial results can be obtained in the case of axisymmetric solutions. We apply these results to some scenarios of potential singularity formation for axi-symmetric solutions, and obtain extensions of results in a recent paper by Chen, Strain, Tsai and Yau [4].

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Let$G$denote a totally disconnected locally compact metric abelian group with translation invariant metric$d$and character group$Γ$_{$G$}. The Lipschitz spaces are defined by $$Lip\left( {\alpha ;p} \right) = \left\{ {f \in L^p \left( G \right):\left\| {\tau _a f - f} \right\|_p = O\left( {d\left( {a,0} \right)^\alpha } \right),a \to 0} \right\},$$ where$τ$_{$a$}$f$:$x$→$f$($x-a$) and α∈(0,1). For a suitable choice of metric it is shown that Lip (α;$p$)⊂$L$^{$r$}($Γ$_{$G$}), where α>1/$p$+1/$r$−1≧0 and 1≦$p$≦2. In the case$G$is compact the corresponding result holds for α>1/$r$−1/2 and$p$>2. In addition for$G$non-discrete the above result is shown to be sharp, in the sense that the range of values of α cannot be extended. The results include classical theorems of S. N. Bernstein, O. Szász and E. C. Titchmarsh.

The main goal of this paper is to present an alternative, real variable proof of the$T$(1)-theorem for the Cauchy integral. We then prove that the estimate from below of analytic capacity in terms of total Menger curvature is a direct consequence of the$T$(1)-theorem. An example shows that the$L$^{∞}-BMO estimate for the Cauchy integral does not follow from$L$^{2}boundedness when the underlying measure is not doubling.