@article{VMUMM_1995_2_a5,
     author = {D. V. Matyukhin},
     title = {On positive-definite biquadratic forms that cannot be represented as the sum of squares of bilinear forms},
     journal = {Vestnik Moskovskogo universiteta. Matematika, mehanika},
     pages = {29--33},
     publisher = {mathdoc},
     number = {2},
     year = {1995},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/VMUMM_1995_2_a5/}
}