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. 2022 Sep 30;29(7):4479–4555. doi: 10.1007/s11831-022-09752-5

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© The Author(s) 2022

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Fig. 7 — An example of grids (a) of three hierarchical levels for $\hat{d} = 1$ . The univariate B-splines of degree 3 defined on level 0, 1 and 2 are shown in (b–d), respectively. All internal knots have multiplicity one