Question: > 
> Chika Ofili, a 12-year-old Nigerian boy based in the United Kingdom, has received the ‘TruLittle Hero Awards’ for discovering a new mathematical formula. [Source](http://www.nigerianeye.com/2019/11/meet-chika-ofili-12-year-old-nigerian.html)
> 
> 
> 

According to the above website (and several others), he envisioned [a simple method to determine if a number is divisible by 7](https://math.stackexchange.com/q/3444466/183815) (which was not included in his textbook), which is amazing on its own!

However, I am bothered by the claim of this being a **new mathematical formula**.

Is this really new?

Answer: 
This and related divisibility tests are *ancient* and well-known to most who have mastered a first course in elementary number theory. They go back at least a couple hundred years, and possibly even much further (e.g. possibly to circa 1200 given Leonardo Pisano (b.k.a. [Fibonacci](https://en.wikipedia.org/wiki/Fibonacci)) discussed tests for divisibility by 7,9,11 in his [Liber Abaci](https://en.wikipedia.org/wiki/Liber_Abaci) (Book of Calculation))

For example, Dickson's [History, v.1 p. 339](https://archive.org/details/historyoftheoryo01dick/page/339) cites a general divisibility test credited to A. L. Crelle in 1844 which has the above test as a special case. I quote Dickson in the 4 bold lines below:

[![enter image description here](https://i.stack.imgur.com/hyKqt.png)](https://i.stack.imgur.com/hyKqt.png)
[![enter image description here](https://i.stack.imgur.com/Fzz7V.png)](https://i.stack.imgur.com/Fzz7V.png)
[![enter image description here](https://i.stack.imgur.com/fD5Fq.png)](https://i.stack.imgur.com/fD5Fq.png)

The prior 3 paragraphs are excerpted from one of my [math.SE posts](https://math.stackexchange.com/a/2063944/242), which has further remarks. 

Questions about such divisibility tests are FAQs in general-level math forums like math.SE and sci.math. There are thousands of posts discussing such in their archives, including many that explain (in modern language) the above general idea used by Crelle, e.g. [here on math.SE](https://math.stackexchange.com/a/2989299/242). Almost everyone who has mastered a course in elementary number theory knows these ideas well (or could easily derive them if need be).

However, except for a few common cases (e.g. [casting out nines and elevens](https://math.stackexchange.com/a/16015/242)) such divisibility tests generally are not named, nor are they attributed to anyone (which is typical for very old elementary results that are easier to (re)derive than to locate citations for - even more so for results like this that are less useful nowadays given the ubiquity of calculators and computers)
This was published in 1965 as rule no 2 for divisibility by 7 in Vedic Maths. Here’s a blog post from 2009 about it <http://blogannath.blogspot.com/2009/10/vedic-mathematics-lesson-23.html?m=1>

Edit: here’s a Wikipedia link showing Vedic Maths was published in 1965. <https://en.m.wikipedia.org/wiki/Vedic_Maths>