\definecolor{A}{RGB}{190,0,60}\color{A}\fbox{A1.} Find all f:R→R such that \definecolor{A}{RGB}{80,0,200}\color{A} x^4+y^4+z^4\ge f(xy)+f(yz)+f(zx)\ge xyz(x+y+z)holds for all a,b,c∈R.Proposed by [color=#FFFF00]usjl.
[color=#B6D7A8]#1733 inequalitiesAM-GMalgebrafunctional equationIMOC