Ray transform on Sobolev spaces of symmetric tensor fields, I: Higher order Reshetnyak formulas

For an integer $rge0$, we prove the $r$th order Reshetnyak formula for the ray transform of rank $m$ symmetric tensor fields on $mathbb{R}^n$. Certain differential operators $A^{(m,r,l)} (0le lle r)$ on the sphere $mathbb{S}^{n-1}$ are main ingredients of the formula. The operators are defined by an algorithm that can be applied for any $r$ although the volume of calculations grows fast with $r$. The algorithm is realized for small values of $r$ and Reshetnyak formulas of orders $0,1,2$ are presented in an explicit form.