Creo que la desigualdad es correcta. Sea $$ f(x)=\sum_{n=0}^\infty(a_n\cos(n\pi x)+b_n\sin(n\pi x)). $$ Esto se debe a que $\{\cos(n\pi x),\sin(n\pi x)\}_{n=0}^\infty$ es denso en $C^1((0,1))\cap C([0,1])$ . Observando que, para los enteros $m, n$ , \begin {eqnarray} & \int_0 ^1 \cos ^2(n \pi x)dx= \int_0 ^1 \sin ^2(n \pi x)dx= \frac12 , \\ & \int_0 ^1 \cos (n \pi x) \sin (m \pi x)dx=0, \\ & \int_0 ^1 \cos (n \pi x) \cos (m \pi x)dx=0 \text { si }m \neq n, \\ & \int_0 ^1 \sin (n \pi x) \sin (m \pi x)dx=0 \text { si }m \neq n, \end {eqnarray} tenemos \begin {eqnarray} \int_0 ^1(f(x))^2dx&=& \int_0 ^1( \sum_ {n=0}^ \infty (a_n \cos (n \pi x)+b_n \sin (n \pi x)))^2dx \\ &=& \int_0 ^1 \sum_ {m,n=0}^ \infty (a_m \cos (m \pi x)+b_m \sin (m \pi x))(a_n \cos (n \pi x)+b_n \sin (n \pi x))dx \\ &=& \frac {1}{2} \sum_ {n=0}^ \infty (a_n^2+b_n^2), \\ \int_0 ^1(f'(x))^2dx&=& \int_0 ^1( \sum_ {n=0}^ \infty (-n \pi a_n \sin (n \pi x)+n \pi b_n \cos (n \pi x)))^2dx \\ &=& \int_0 ^1 \sum_ {m,n=0}^ \infty mn \pi ^2(-a_m \sin (m \pi x)+b_m \cos (m \pi x))(-a_n \sin (n \pi x)+b_n \cos (n \pi x))dx \\ &=& \frac {1}{2} \sum_ {n=0}^ \infty n^2 \pi ^2(a_n^2+b_n^2). \end {eqnarray} Así, \begin {eqnarray} && \int_0 ^1(f(x))^2dx+ \int_0 ^1(f'(x))^2dx- \frac12 (f(0))^2 \\ &=& \frac12\sum_ {n=0}^ \infty (n^2 \pi ^2+1)(a_n^2+b_n^2)- \frac12 ( \sum_ {n=0}^ \infty a_n)^2 \\ & \ge & \frac12\left ( \sum_ {n=0}^ \infty (n^2 \pi ^2+1)a_n^2-( \sum_ {n=0}^ \infty a_n)^2 \right )+ \frac12\sum_ {n=0}^ \infty (n^2 \pi ^2+1)b_n^2. \end {eqnarray} Creo que $$ \sum_{n=0}^\infty(n^2\pi^2+1)a_n^2-(\sum_{n=0}^\infty a_n)^2\ge 0$$ y ahora no tengo tiempo para probarlo. Así que $$ \int_0^1(f(x))^2dx+\int_0^1(f'(x))^2dx\ge\frac12(f(0))^2. $$