Is Every Positive Integer Divisible by 2, 3, or 5?

In summary, the conversation discusses a mathematical concept that looks at the divisibility of positive integers by different numbers. The conversation delves into finding the sum of all positive integers that are not relatively prime to 2, 3, and 5. The conversation also presents a formula to simplify the process of finding this sum. The main goal is to prove that the sum tends to 0 as the number of terms increases.
  • #1
marcusmath
16
0
I really didn't know how to word the title so sorry if it's a little confusing.
And I didn't know whether to post this in number theory or not but ah well.

The other day, I started thinking about this and I was just wondering if it had been done before or if it's even correct;

Half of all positive integers are divisible by 2,
A third are divisible by 3,
A quarter by 4 etc.

Now, of the third that are divisible by 3, a half of those numbers are also divisible by 2.
So 1/6 of positive integers are divisible by 3 and not 2.

I continued this trend to find;
[tex]\frac{1}{2}[/tex] of p. ints are divisible by 2
[tex]\frac{1}{3}\left(1-\frac{1}{2}\right)[/tex] are divisible by 3 and not 2
[tex]\frac{1}{5}\left(1-\left(\frac{1}{2}+\frac{1}{3}\left(1-\frac{1}{2}\right)\right)\right)[/tex] are divisible by 5 and not 3 or 2
[tex]\frac{1}{7}\left(1-\left(\frac{1}{2}+\frac{1}{3}\left(1-\frac{1}{2}\right)+\frac{1}{5}\left(1-\left(\frac{1}{2}+\frac{1}{3}\left(1-\frac{1}{2}\right)\right)\right)\right)\right)[/tex] are divisible by 7 and not 5, 3 or 2

If I sum all these to infinity, it should equal 1 as it will then cover all positive integers.

So could you by any chance verify the truth of this statement,

[tex]\sum^{\infty}_{p_{0}=2}\frac{1}{p_{0}}\left(1-\sum^{p_{0}-1}_{p_{1}=2}\frac{1}{p_{1}}\left(1-\sum^{p_{1}-1}_{p_{2}=2}\frac{1}{p_{2}}\left(...\right)\right)\right) = 1[/tex]

I know this is quite awkward looking and I don't even know if I've used correct notation, I just sort of scrawled it down as I was thinking it.
 
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  • #2
To make it a little easier: the sum of "divisible by 2" + "not divisible by 2 but divisible by 3" + "not divisible by 2 or 3 but divisible by 5" is "divisible by 2, 3, or 5" which is "not relatively prime to 2, 3, and 5":

[tex]f_1 = 1/2[/tex]
[tex]f_2 = 1/2 + 1/2\cdot1/3 = 1 - 1/2\cdot2/3[/tex]
[tex]f_3 = 1/2 + 1/2\cdot1/3 + 1/2\cdot2/3\cdot1/5 = 1 - 1/2\cdot2/3\cdot4/5[/tex]
. . .
[tex]f_k = 1 - \sum_{q\in\mathbb{P},q\le p_k}\left(1-\frac1p\right)[/tex]

Now instead of lots of sums, you only have one. All you need to show is that the sum in question tends to 0 as k goes to infinity (which is true).
 

1. What is a fraction?

A fraction is a mathematical expression that represents a part of a whole number. It consists of a numerator (the number above the line) and a denominator (the number below the line).

2. How do you find the fraction of a number?

To find the fraction of a number, you multiply the number by the fraction. For example, to find 1/4 of 12, you would multiply 12 by 1/4, which equals 3.

3. How do you convert a fraction to a decimal?

To convert a fraction to a decimal, you divide the numerator by the denominator. For example, to convert 3/4 to a decimal, you would divide 3 by 4, which equals 0.75.

4. How do you add and subtract fractions?

To add or subtract fractions, you must first find a common denominator. Then, you add or subtract the numerators and keep the common denominator. For example, to add 1/3 and 1/4, you must first find a common denominator of 12. The fractions then become 4/12 and 3/12, which can be added to equal 7/12.

5. How do you simplify a fraction?

To simplify a fraction, you must divide the numerator and denominator by their greatest common factor. This will result in a fraction with smaller numbers. For example, to simplify 12/16, you would divide both numbers by 4 to get 3/4.

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