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Instead of considering additional structure, it is also possible to take into account various notions of manifold, especially piecewise linear (PL) and topological manifolds. This gives rise to bordism groups Ω P L ( X ) , Ω T O P ( X ) {\displaystyle \Omega _{*}^{PL}(X),\Omega _{*}^{TOP}(X)} , which are harder to compute than the differentiable variants.[citation needed]