Roy Lewallen wrote:
"But the avreage value of a sine wave, assuming no DC offset, is zero,
not 0.637 times the peak value."

Yes. The value of any undistorted sine wave which is symmetrical about
the zero axis is zero over one complete cycle.

0.637 and 0.707 are half-cycle values which are quoted as the average
and rms fractions of the peak value of a sine wave in references
containing such trivia. I just remember tthat the average in this case
is about 0.9x the rms value.

Best regards, Richard Harrison, KB5WZI