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Sep 14, 2019 at 14:54 comment added Pietro Majer I like them all, yet I think the true proof of the plain density claim is number 2. We may also apply the lemma to $u\mapsto u'(0)$ as a discontinuous linear functional say on $H^2\cap H^1_0([0,1/10])$ with the $H^1$-norm. As a consequence, arbitrarily close to any function $v\in H^2(0,1)$ wrto the $H^1$ distance there is a function $w\in H^2(0,1)$ with arbitrary derivative at $0$ (and with $v=w$ on $[1/10,1]$ so that we can also perturb on the other side independently and find an approximation of $v$ in $V$).
Sep 13, 2019 at 23:37 vote accept Trần Quang Minh
Sep 13, 2019 at 23:34 history answered Jochen Glueck CC BY-SA 4.0