Two logicians place cards on their foreheads so that what is written on the card is visible only to the other logician. Consecutive positive integers

have been written on the cards. The following conversation ensues
A: "I don't know my number." B: "I don't know my number." A: "I don't know my number." B: "I don't know my number." ... n statements of ignorance later ... A or B: "I know my number."

What is on the card and how does the logician know it?

Integers Solution

lib/DbaDatabase.php:134: Warning: dba_replace() [<a href='function.dba-replace'>function.dba-replace</a>]: You cannot perform a modification to a database without proper access