For natural numbers $r$ and $n$ and a real number $a$ we construct a natural vector bundle $T^{\left(r\right),a}$ over $n$-manifolds such that $T^{\left(r\right),0}$ is the (classical) vector tangent bundle $T^{\left(r\right)}$ of order $r$. For integers $r\ge 1$ and $n\ge 3$ and a real number $a<0$ we classify all natural operators $T_{|M_n}⇝T{T}^{\left(r\right),a}$ lifting vector fields from $n$-manifolds to $T^{\left(r\right),a}$.

For integers $r\ge 2$ and $n\ge 2$ a complete classification of all natural operators $A:T_{|M_n}⇝T{\left(J^r{T}^{*}\right)}^{*}$ lifting vector fields to vector fields on the natural bundle ${\left(J^r{T}^{*}\right)}^{*}$ dual to $r$-jet prolongation $J^r{T}^{*}$ of the cotangent bundle over $n$-manifolds is given.

All natural operators T ↝ T(T ⊗ T*) lifting vector fields X from n-dimensional manifolds M to vector fields B(X) on the bundle of affinors ™ ⊗ T*M are described.

We study the problem of how a map f:M → ℝ on an n-manifold M induces canonically an affinor $A\left(f\right):T{T}^{\left(r\right)}M\to T{T}^{\left(r\right)}M$ on the vector r-tangent bundle $T^{\left(r\right)}M=\left(J^r(M,ℝ)₀\right)*$ over M. This problem is reflected in the concept of natural operators $A:T_{|ℳfₙ}^{(0,0)}⇝T^{(1,1)}{T}^{\left(r\right)}$. For integers r ≥ 1 and n ≥ 2 we prove that the space of all such operators is a free (r+1)²-dimensional module over ${}^{\infty }\left(T^{\left(r\right)}ℝ\right)$ and we construct explicitly a basis of this module.

For natural numbers $r\ge 2$ and $n$ a complete classification of natural transformations $A:T{T}^{\left(r\right)}\to T{T}^{\left(r\right)}$ over $n$-manifolds is given, where $T^{\left(r\right)}$ is the linear $r$-tangent bundle functor.

We give an expository account of a Weierstrass type representation of the non-zero constant mean curvature surfaces in space and discuss the meaning of the representation from the point of view of partial differential equations.

We study some properties of the polar curve $P_l^{ℱ}$ associated to a singular holomorphic foliation $ℱ$ on the complex projective plane ${\mathbb{P}}^2$. We prove that, for a generic center $l\in {\mathbb{P}}^2$, the curve $P_l^{ℱ}$ is irreducible and its singular points are exactly the singular points of $ℱ$ with vanishing linear part. We also obtain upper bounds for the algebraic multiplicities of the singularities of $ℱ$ and for its number of radial singularities.

Our aim is to study the principal bundles determined by the algebra of quaternions in the projective model. The projectivization of the conformal model of the Hopf fibration is considered as example.