This is a different post from my usual style – there will be no pictures, and just the one link which I feel must be shared and which will feature at the end of the post.


It is inevitable when writing about the number 40 that there will be considerable overlap with the detail contained in Derrick Niedermann’s wonderful book Number Freak but I hope that some of the stuff I come with is new. One of the things Niedermann talks about is the use of forty in ancient times to denote ‘a large number’ in which he context he mentions various biblical references and the tale of Ali Baba and the Forty Thieves – which reference particularly appeals as I am the proud owner of both a four volume boxed set of the complete 1,001 nights and a Folio Society edition of the highlights.


It can be expressed as the sum of a square and a triangle in two different ways: Two squared added to the eighth triangle number or five squared added to the fifth triangle number (note 8+2 = 10 and 5+5 = 10).

It is both double and quadruple a tetrahedral number (10= 1+3+6 = the sum of the first three triangle numbers and 20 + 1+3+6+10 = the sum of the first four triangle numbers).

It is the sum of the fourth triangle and the fourth pyramid number (10 = 1+2=3+4 and 30 = 1+4+9+16) and it is also thus the sum of third tetrahedral number and the fourth pyramid number.

Another connection of two fours and forty is that four squared plus 4 factorial = 16 + 24 = 40.


I have written about the number 40 because today is my 40th birthday and I thought this would be a fun way to commemorate the landmark for followers of my blog.


I am sharing one link with this post, from Autsim Mom, who will be visiting this country shortly. This post was first published before I had started following that blog and I am delighted to share it now.

Author: Thomas

I am branch secretary of NAS West Norfolk and #actuallyautistic (diagnosed 10 years ago at the comparatively advanced age of 31). I am a keen photographer, so that most of my own posts contain photos. I am a keen cricket fan and often write about that subject. I also focus a lot on politics and on nature.

4 thoughts on “Forty”

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