Approximation and orthogonality in Sobolev spaces on a triangle

Approximation by polynomials on a triangle is studied in the Sobolev space $W_2^r$ that consists of functions whose derivatives of up to $r$-th order have bounded $L^2$ norm. The first part aims at understanding the orthogonal structure in the Sobolev space on the triangle, which requires explicit construction of an inner product that involves derivatives and its associated orthogonal polynomials, so that the projection operators of the corresponding Fourier orthogonal expansion commute with partial derivatives. The second part establishes the sharp estimate for the error of polynomial approximation in $W_2^r$, when $r = 1$ and $r=2$, where the polynomials of approximation are the partial sums of the Fourier expansions in orthogonal polynomials of the Sobolev space.