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Biharmonic homogeneous polynomial maps between spheres

In this paper we first prove a characterization formula for biharmonic maps in Euclidean spheres and, as an application, we construct a family of biharmonic maps from a flat $2$-dimensional torus $\mathbb{T}$ into the $3$-dimensional unit Euclidean sphere $\mathbb{S}^3$. Then, for the special case of maps between spheres whose components are given by homogeneous polynomials of the same degree, we find a more specific form for their bitension field. Further, we apply this formula to the case when the degree is $2$, and we obtain the classification of all proper biharmonic quadratic forms from $\mathbb{S}^1$ to $\mathbb{S}^n$, $n \geq 2$, from $\mathbb{S}^m$ to $\mathbb{S}^2$, $m \geq 2$, and from $\mathbb{S}^m$ to $\mathbb{S}^3$, $m \geq 2$.