"1 is non-prime by definition."
dammit!
dammit!
I have seen tables of prime counts produced in the 1950s and 1960s that specified whether they counted 1 as a prime, and some did count it.
Going through school I'd be told 1 was/not prime differently by different teachers. Always seemed pretty arbitrary to me; like 'They' ought just to decide!
What we seem to be hearing here is that They had, just another classic case of this information taking 50 years to filter down the education system.. (I wonder how many planets they teach there as being in our solar system?)
Your comment got me to look up the number of objects considered planets over the years, this has ranged from 5-23. Data since 1543 is available here: https://dx.doi.org/10.1038%2Fscientificamerican0107-34
For fun I did a linear regression of year vs # planets and found that the while the slope was 1.8 planets/century, this was not significantly different from zero. The evidence appears to be consistent with a constant number of planets.
"The way mathematicians have viewed the number one (unity, the monad) has changed throughout the years. Most of the early Greeks did not view one as a number, but rather as the origin, or generator, of number. Around the time of Simon Stevin (1548-1620), one (and zero) were first widely viewed as numbers. This created a period of confusion about whether or not the number one was prime. In this dynamic survey, we collect a cornucopia of sources which deal directly with the "question what is the smallest prime?" The goal is to create a source book for studying the history of the definition of prime, especially as applied to the number one."