I thought I was bad with mental mathematical gymnastics, but this one from today's Column 8 beats anything I do:

A bad case of indivisible insomnia. ''I have a digital clock on my bedside table,'' writes Barry Ford of Tamworth. ''If I awaken during the night and take note of the time I cannot get back to sleep until I have mentally calculated whether the number showing is a prime number or not.''

Or maybe not… I did wake up early one morning not so long ago trying to work out the square root of i, the imaginary square root of -1

Related articles, courtesy of Zemanta:

• Prime numbers visualised