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Turtle's troglodytes

Posted 05-13-2009 at 07:07 AM by Ben
Updated 05-13-2009 at 08:35 AM by Ben
Let me just remind you what these are: in his blog., Turtle asserted that, for any integer n = abcde (where we may have, say a=b) then n \equiv a+b+c+d+e \mod 9.

Now I confess I haven't read the whole article, so for all I know this may have been covered, but the proof of this is laughably easy.

Take an integer like, oh I dunno, say 467. Historically this notation means 400+60+7. So any integer can be written as 10^na+ 10^{n-1}b+10^{n-2}c + \cdots n.

It is true, almost by definition that 10^n \equiv 1 \mod 9 for any n, so it follows immediately that

n=abcde = 10^na+ 10^{n-1}b+10^{n-2}c + \cdots +n \equiv 1a+1b+1c +\cdots +1n \mod 9.

The very nice thing about this is that we see, also immediately, that, say abc \equiv bac \equiv cab, \cdots \equiv a+b+c \mod 9 by commutativity and associativity of arithmetic addition.
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  1. Old
    Turtle's Avatar
    Thanks for reading as much as you did Ben. Double thanks for the proof two...erhm...too. If you do read on, and it's a rather dauntingly long thread I admit, then we might get to some categorizng. I have a smaller spin-off thread that may give a quicker idea of some things I have done with these residues and obviously I hope it's something you have never seen.
    katabatak powers
    Thanks again for bloggin' me.
    permalink
    Posted 05-13-2009 at 09:37 AM by Turtle Turtle is offline
 


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