We leave the details to the reader.
We leave to the reader the proof that f is indeed self-adjoint, and not merely symmetric.
We leave it to the reader to verify that...... [Note that the it is necessary here.]
For the convenience of the reader, we repeat the main points. [Or: For the reader's convenience]
The reader is cautioned that our notation is in conflict with that of .
The reader is assumed to be familiar with elementary K-theory.
For more details we refer the reader to .
The interested reader is referred to  for further information. [Note the double r in referred.]
At this point, the reader is urged to review the definitions of......
It may be worth reminding the reader that......
The reader might want to compare this remark with [2, Cor. 3].