@article{MS2_1965__17_165924,
     author = {Jaake Peetre},
     title = {Some {Remarks} on {Continuous} {Orthogonal} {Expansions,} and {Eigenfunction} {Expansions} for {Positive} {Self-Adjoint} {Elliptic} {Operators} with {Variable} {Coefficients.}},
     journal = {Mathematica Scandinavica},
     pages = {56--64},
     publisher = {mathdoc},
     volume = {17},
     year = {1965},
     zbl = {0148.13002},
     url = {http://geodesic.mathdoc.fr/item/MS2_1965__17_165924/}
}