title: what does this notation mean <x1,x2>?
body: <p>I was reading about k-strong convexity and came a across an equation like this</p>
  
  <p>f(x_1) >= f(x_2) + &lt;\Delta f(x_2), x_1-x_2> + (k/2)*(||x_1-x_2||^2)</p>
  
  <p>Could some one explain what the notation &lt;> means?</p>
  
  <p>Sorry for the errors with latex.</p>