de Rham cohomology of a finite disjoint union ($H_{\mathrm{dR}}^p({⨆}_{j=1}^k{M}_j)\cong {⨁}_{j=1}^k{H}_{\mathrm{dR}}^p(M_j)$)

Restriction of differential forms to the finitely many open and closed components gives an isomorphism of cochain complexes ${\Omega }^{\ast }(⨆M_j)\cong {⨁}_j{\Omega }^{\ast }(M_j)$. Piecewise forms glue because there are no overlaps, and the exterior derivative acts componentwise. Taking the quotient of closed differential forms by exact differential forms proves the displayed de Rham cohomology isomorphism. For an infinite disjoint union, forms and cohomology give products instead.