Mureşan, Anton S. “Some Properties of Solutions of Equation \(\Delta ^4u=0\) (II)”.
Anal. Numér. Théor. Approx.
, vol. 10, no. 2, Aug. 1981, pp. 201-4,
https://ictp.acad.ro/jnaat/journal/article/view/1981-vol10-no2-art9
.