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The symbol $\Delta u$ represents the Laplacian of the scalar field $u$. It is a compact notation for the sum of the unmixed second partial derivatives of $u$ with respect to all coordinate directions.

Conditions: $u$ is a scalar function of multiple variables.; The coordinates are orthogonal Cartesian coordinates (implied by the simple sum of second derivatives).; The notation generalizes the 2D case to $n$ dimensions.

The symbol $\Delta u$ represents the Laplacian of the scalar field $u$. It is a compact notation for the sum of the unmixed second partial derivatives of $u$ with respect to all coordinate directions.

Conditions: $u$ is a scalar function of multiple variables.; The coordinates are orthogonal Cartesian coordinates (implied by the simple sum of second derivatives).; The notation generalizes the 2D case to $n$ dimensions.

The two-dimensional Laplace equation is defined as the sum of the second partial derivatives of a scalar function $u$ with respect to the spatial coordinates $x$ and $y$ equaling zero. It is presented on a rectangular domain defined by $0 < x < a$ and $0 < y < b$.

Conditions: The function $u$ is a scalar field depending on two independent spatial variables $x$ and $y$.; The domain is an open rectangle in the $xy$-plane.; The equation is homogeneous (equals zero).

The two-dimensional Laplace equation is defined as the sum of the second partial derivatives of a scalar function $u$ with respect to the spatial coordinates $x$ and $y$ equaling zero. It is presented on a rectangular domain defined by $0 < x < a$ and $0 < y < b$.

Conditions: The function $u$ is a scalar field depending on two independent spatial variables $x$ and $y$.; The domain is an open rectangle in the $xy$-plane.; The equation is homogeneous (equals zero).