Decomposing with smooth sets

A subset of Euclidean space will be said to be $n$-smooth if it has an $n$-dimensional tangent plane at each of its points. Let ${\frak d}_n$ denote the least number $n$-smooth sets into which $n+1$-dimensional Euclidean space can be decomposed. For each $n$ it is shown to be consistent that ${\frak d}_n > {\frak d}_{n+1}$. Moreover, the inequalities ${\frak d}_{n+1}^+ \geq${\frak d}_n$are established where${\frak d}_1$is defined to be the continuum. The cardinal invariant${\frak d}_2$is shown to be the same as the least$κ$such that each continuous function from the reals to the reals can be decomposed into$κ$ differentiable functions.

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