@article{MS2_1964__14_165873,
     author = {Sigbjorn Halvorsen},
     title = {On the {Quadratic} {Integrability} of {Solutions} of d2x/dt2 + f(t)x = 0.},
     journal = {Mathematica Scandinavica},
     pages = {111--119},
     publisher = {mathdoc},
     volume = {14},
     year = {1964},
     url = {http://geodesic.mathdoc.fr/item/MS2_1964__14_165873/}
}