There has been quite a lot of feedback, some quite vitriolic (mostly on Twitter), about the "discovery" referred to in the previous blog post so I thought I would clarify a few things:
No - it's not strictly "New" Maths but...
It was discovered independently by an 11 year-old boy who was very excited to share his discovery so it was NEW to him.
None of the teachers at his school - nor anyone they consulted - had seen it before so it was NEW to all of them, too.
I had only seen something like it as an exercise in Modular Arithmetic (and not as a divisibility test) so when I was asked to come up with a watertight proof that 11 year olds could understand (see below) that was NEW to me, too.
At the time, I did an extensive internet search and could only find one small reference tucked away inside a Wikipedia page - without the proof in my original blog...
...and, finally, I never actually said it was "NEW" Maths. There was always a question mark in the title (and in the first line of the article) and I am very grateful to those who have answered that question by pointing out other places where they have seen this divisibility test before - especially those who did it with good grace!
Let us all celebrate creativity in Mathematics and be grateful that there are 11 year olds out there who do this sort of thing for fun!
Simon Ellis
20th November 2019
Hey! I think this is all very great and I'm glad that people are learning more math and that Chika is getting the chance to feel how wonderful it is to figure something out through exploration and share it with others who are excited to use it.
I wanted to add something that I thought people who like Chika's Test might appreciate. The first is that if you are comfortable with the 2 digit multiples of 7, then you can make Chika's rule go faster by doing it two digits at a time and using 4 as the multiplier instead of 5. Also, reduce by as many multiples of 7 as you can, whenever you can to keep the arithmetic…