When does $\|x+y\|=\|x\|+\|y\|?$

Question

Let $(V_\mathbb R,\langle,\rangle)$ be an inner product space. I'm trying to see for $x,y\in V$ when does $\|x+y\|=\|x\|+\|y\|?$

• Let $\|x+y\|=\|x\|+\|y\|$

  Squaring both sides, $\langle x+y,x+y\rangle=\langle x,x\rangle+\langle y,y\rangle+2\|x\|.\|y\|\\\ge\langle x,x\rangle+\langle y,y\rangle+2\langle x,y\rangle\\=\langle x,x\rangle+\langle x,y\rangle+\langle x,y\rangle+\langle y,y\rangle\\=\langle x,x+y\rangle+\langle x+y,y\rangle...(*)\\\implies\langle x+y,x+y\rangle-\langle x,x+y\rangle\ge\langle x+y,y\rangle\\\implies0\ge\langle x,0\rangle+\langle y,y\rangle\text{ due to linearity}\\\implies y=0$

  Again from $(*),\langle x+y,x+y\rangle-\langle x+y,y\rangle\ge\langle x,x+y\rangle\\\implies 0\ge\langle x,x\rangle+\langle0,y\rangle\\\implies x=0$

---

Where did I go wrong?

Answer 1

The third line in your formulas can be written (if you use that $\langle x,y\rangle=\langle y,x\rangle$) as $\langle x+y,x+y\rangle$. So you have equality in the inequality in your second line. This implies $$ \langle x,y\rangle=\|x\|\,\|y\|, $$ i.e. equality in Cauchy-Schwarz. So $x$ and $y$ are colinear.

Regarding your mistake, the two sides of the inequality in your first "implies" are equal. So your second "implies" is wrong.

Answer 2

Suppose $||x+y|| = ||x|| + ||y||$. Then we have $$\begin{align*} \langle x + y, x + y \rangle &= \langle x, x + y \rangle + \langle y, x + y \rangle\\ &= \langle x, x \rangle + 2\langle x, y \rangle + \langle y, y \rangle\\ ||x+y||^2&= ||x||^2 + 2\langle x, y \rangle + ||y||^2\\ \end{align*}$$

But from the assumption we have $||x+y||^2 = (||x|| + ||y||)^2 = ||x||^2 + 2||x||\,||y|| + ||y||^2\\$. By the Cauchy–Schwarz inequality, this means x and y are linearly dependent, and specifically $x=cy$ for $c\ge 0$ (notice the absolute value around the inner product in the Cauchy-Schwarz inequality).

I just realized I never addressed your question. The first implication is incorrect, since $\langle x+y,x+y\rangle-\langle x,x+y\rangle=\langle y,x+y\rangle$. In a real inner product space, your "inequality" is an equality, and in a complex vector space it is not safe to assume inequality.