2
$\begingroup$

Is a sufficient condition for a $2\times 2$ matrix $$\left(\begin{array}{cc}a&b\\b&d\end{array}\right)$$ to be positive definite that $a >0$ and $ad > b^2$ ?

  • 0
    Do you mean, a $2\times 2$ matrix $$\left(\begin{array}{cc}a&b\\d&c\end{array}\right)\ ?$$2012-04-19
  • 0
    Welcome to math.SE. For future reference, soft-questions are more non-mathematical in nature. I've changed to more appropriate tags.2012-04-19
  • 0
    @ArturoMagidin: The $c$ and $d$ are interchanged.2012-04-19
  • 1
    @Robb: Arturo wrote it like you said it. If you interchange $c$ and $d$, then they're not clockwise.2012-04-19
  • 0
    @ArturoMagidin: Actually you are correct.2012-04-19
  • 0
    @Robb: Which is it? The one I wrote in the comment, or the one I put in the question?2012-04-19
  • 0
    @ArturoMagidin: The one you put in the question.2012-04-19

4 Answers 4