Just when you thought you were feeling safe ....
We had an idea that there might be some numbers that had a ghostly quality. The Devils Number 666 came to mind, but that didn't really fit the bill.
Then came that Aha! moment. There are special numbers of many kinds, and amongst them are the twin primes. Numbers like 11 and 13, 41 and 43, 59 and 61 are examples of twin primes, They are all primes numbers that differ by 2. One of the unsolved problems of maths is the hypothesis that there are an infinite number of twin primes and this gives them quite a bit of notoriety. But in between every twin pair is a number that has never been considered to be interesting. That's where we felt that the ghostly numbers would be hiding and, lo and behold, these numbers do have an intersting property. We'll let you discover what is the property that all ghostly numbers (apart from the smallest obne) share.
Maybe your studnets would like to follow this one up. If so, we'd love to hear from them!