Can linear transformations which reduces the dimensionality of vectors preserve the Euclidean distances between them?

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$y^{(i)}$ = M$x^{(i)}$, where $x^{(i)}$ is in 3D, $y^{(i)}$ is in 2D and Euclidean distance between $y^{(i)}$ and $y^{(j)}$ is same as $x^{(i)}$ and $x^{(j)}$. Does M exist?

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No. Since the dimension is reduced there is a non-trivial nullspace. Choose any non-zero vector in that nullspace and it will have length greater than zero, but all vectors in the nullspace will be zero after the transformation and thus have length zero so the length is not preserved.