I want to know how to prove that the below matrix is invertible
\begin{pmatrix} 1 & \frac { 1 }{ 2 } & ... & \frac { 1 }{ n } \\ \frac { 1 }{ 2 } & \frac { 1 }{ 3 } & ... & \frac { 1 }{ n+1 } \\ ... & ... & ... & ... \\ \frac { 1 }{ n } & \frac { 1 }{ n+1 } & ... & \frac { 1 }{ 2n-1 } \end{pmatrix}
and that the inverse matrix has integer entries.