Abstract. Navier-Stokes flows are found on R 2 that decay in time more rapidly than observed in general. The decay rate is determined in accordance with the order of symmetry with respect to the action of dihedral groups of orthogonal transformations. Contrary to the previous work [13], the basic existence result is proved with no restriction on the size of initial data. Our result extends that of [3] under di¤erent assumptions on the initial data. Unlike [3], the proofs are all carried out without using estimates for the associated vorticity transport equations. Key Words and Phrases. Navier-Stokes system, Cauchy problem, Group symmetry, Asymptotic behavior, Weighted estimate.2000 Mathematics Subject Classification Numbers. 35Q30, 76D05.
Statement of the resultsThis paper studies the Cauchy problem for the Navier-Stokes system in R 2 :Here, x ¼ ðx 1 ; x 2 Þ A R 2 , t b 0, q t ¼ q=qt; u ¼ uðx; tÞ ¼ ðu k ðx; tÞÞ 2 k¼1 and p ¼ pðx; tÞ denote unknown velocity and pressure, respectively; and a ¼ aðxÞ is a given initial velocity. The kinematic viscosity is normalized. The standard notation is used for di¤erential operators and function spaces and the summation convention is employed for repeated indices.We are interested in finding a (unique) solution u to (1.1) that satisfies kuðtÞk r a cð1 þ tÞ Àðmþ1Þ=2Àð1À1=rÞ ð1 a r a yÞ ð1:2Þ for some m A N U f0g. (Here and in what follows k Á k r denotes the L r -norm.) In view of the structure of solutions to parabolic equations, it is reasonable to guess that such solutions would decay su‰ciently rapidly as t þ jxj ! y; and our purpose is to specify a class of initial data that provide us with solutions satisfying (1.2).