Mollification in Strongly Lipschitz Domains with Application to Continuous and Discrete De Rham Complexes

We construct mollification operators in strongly Lipschitz domains that do not invoke non-trivial extensions, are L p stable for any real number <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mrow> <m:mi>p</m:mi> <m:mo>∈</m:mo> <m:mo>[</m:mo> <m:mn>1</m:mn> <m:mo>,</m:mo> <m:mi>∞</m:mi> <m:mo>]</m:mo> </m:mrow> </m:math> p∈ [1,∞ ] , and commute with the differential operators ∇, <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mrow> <m:mi>∇</m:mi> <m:mo>×</m:mo> </m:mrow> </m:math> ∇ × , and <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mrow> <m:mi>∇</m:mi> <m:mo>·</m:mo> </m:mrow> </m:math> ∇ · . We also construct mollification operators satisfying boundary conditions and use them to characterize the kernel of traces related to the tangential and normal trace of vector fields. We use the mollification operators to build projection operators onto general H 1 -, <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mrow> <m:mi>H</m:mi> <m:mo>(</m:mo> <m:mi> curl </m:mi> <m:mo>)</m:mo> </m:mrow> </m:math> H(curl) - and <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mrow> <m:mi>H</m:mi> <m:mo>(</m:mo> <m:mi> div </m:mi> <m:mo>)</m:mo> </m:mrow> </m:math> H(div) -conforming finite element spaces, with and without homogeneous boundary conditions. These operators commute with the differential operators ∇, <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mrow> <m:mi>∇</m:mi> <m:mo>×</m:mo> </m:mrow> </m:math> ∇ × , and <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mrow> <m:mi>∇</m:mi> <m:mo>·</m:mo> </m:mrow> </m:math> ∇ · , are L p -stable, and have optimal approximation properties on smooth functions.