Proof: Integer Divisibility by 3 via Polynomials

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The discussion presents a proof that an integer is divisible by 3 if and only if the sum of its digits is divisible by 3. It establishes that for a polynomial function P(x), the evaluation at 10 modulo 3 is equivalent to the evaluation at 1 modulo 3, due to the property that 10 is congruent to 1 modulo 3. This leads to the conclusion that the integer N, represented by its digits, is congruent to the sum of its digits modulo 3. The proof is supported by the concept of ring homomorphism, which maintains the operations of addition and multiplication under modulo conditions. Thus, the divisibility rule for 3 is confirmed through polynomial evaluation and modular arithmetic principles.
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Homework Statement
Establish the following divisibility criteria:
An integer is divisible by ## 3 ## if and only if the sum of its digits is divisible by ## 3 ##.
Relevant Equations
None.
Proof:

Let ## P(x)= \Sigma^{m}_{k=0} a_{k} x^{k} ## be a polynomial function.
Then ## N=a_{m}10^{m}+a_{m-1}10^{m-1}+\dotsb +a_{1}10+a_{0} ## for ## 0\leq a_{k}\leq 9 ##.
Since ## 10\equiv 1\pmod {3} ##, it follows that ## P(10)\equiv P(1)\pmod {3} ##.
Note that ## N\equiv (a_{m}+a_{m-1}+\dotsb +a_{1}+a_{0})\pmod {3} ##.
Thus ## 3\mid N\Leftrightarrow N\equiv 0\pmod {3}\Leftrightarrow P(10)\equiv 0\pmod {3}\Leftrightarrow P(1)\equiv 0\pmod {3} ##.
This means ## 3\mid P(1)\Leftrightarrow 3\mid (a_{m}+a_{m-1}+\dotsb +a_{2}+a_{1}+a_{0}) ##.
Therefore, an integer is divisible by ## 3 ## if and only if the sum of its digits is divisible by ## 3 ##.
 
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Though It is not different from your answer,
10^n \equiv 1(\mod 3)
Proof
For n=0 ##10^0=1\equiv 1(\mod 3)##
Say n satisfy it ##10^{n+1} \equiv 10 \equiv 1(\mod 3)##
prooved
Say abcd...z a decimal positive integer number
abcd...z \equiv a+b+c+d+...+z(\mod 3)
 
Math100 said:
Homework Statement:: Establish the following divisibility criteria:
An integer is divisible by ## 3 ## if and only if the sum of its digits is divisible by ## 3 ##.
Relevant Equations:: None.

Proof:

Let ## P(x)= \Sigma^{m}_{k=0} a_{k} x^{k} ## be a polynomial function.
Then ## N=a_{m}10^{m}+a_{m-1}10^{m-1}+\dotsb +a_{1}10+a_{0} ## for ## 0\leq a_{k}\leq 9 ##.
Since ## 10\equiv 1\pmod {3} ##, it follows that ## P(10)\equiv P(1)\pmod {3} ##.
Note that ## N\equiv (a_{m}+a_{m-1}+\dotsb +a_{1}+a_{0})\pmod {3} ##.
Thus ## 3\mid N\Leftrightarrow N\equiv 0\pmod {3}\Leftrightarrow P(10)\equiv 0\pmod {3}\Leftrightarrow P(1)\equiv 0\pmod {3} ##.
This means ## 3\mid P(1)\Leftrightarrow 3\mid (a_{m}+a_{m-1}+\dotsb +a_{2}+a_{1}+a_{0}) ##.
Therefore, an integer is divisible by ## 3 ## if and only if the sum of its digits is divisible by ## 3 ##.
Correct, yes.

You have used ##10\equiv 1\pmod 3 \Longrightarrow P(10)\equiv P(1) \pmod 3.##

The reason why this is true is that modulo is a ring homomorphism. A ring is a structure with addition, multiplication (but without division), and the distributive law, e.g. ##\mathbb{Z}##. A ring homomorphism means it respects the two operations. This means
\begin{align*}
a+b \pmod n &\equiv a\pmod n +b \pmod n \\
a\cdot b \pmod n &\equiv a\pmod n \cdot b \pmod n
\end{align*}
Therefore ##P(x) \pmod n \equiv P(x\pmod n)## by consecutive applications of these two laws.
 
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