What is a "Popa" and who might he be?

Popa is a Swedish term of endearment.
It is used mainly by children for their grandfathers and also implies an older wise man for whom you have affection. Other Popas might be an unmarried uncle or an older man about the village who spends time telling stories to the children.

Our Popa is Dr. A. V. Persson.
Dr. Persson is a retired vascular surgeon who has been writing Popa’s Notes for several years. Originally intended for circulation among his family, Popa's Notes soon acquired devotees outside of this circle. So, Dr. Persson started sending it to his many friends. Recently, he agreed to share Popa's Notes with people interested in Smart Science.

These notes are intended solely for your entertainment. Both Popa and the crew at Smart Science hope you enjoy it.