Number of prime factors

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Discussion Overview

The discussion revolves around the exploration of a function that estimates the average number of prime factors for integers in a specified range. Participants are examining the relationship between this function and the logarithmic function log log x, as well as the implications of counting indistinct prime factors.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant inquires about a function f(x) that could provide the average number of prime factors for integers from 0 to x, similar to how Li(x)/x estimates the likelihood of a number being prime.
  • Another participant suggests using log log x as a potential function for this purpose.
  • A participant reports their calculations for the average number of prime factors for specific values of x (1000, 10,000, 100,000) and questions the accuracy of the log log x approach based on their results.
  • It is noted that a constant factor, approximately 1.03465388, is relevant when counting indistinct prime factors, which may affect the predictions made by log log x.
  • Participants discuss the placement of this constant in the expected number of prime factors per number up to x, suggesting it should be added to log log x.
  • There is a question regarding the derivation of the constant, with one participant stating that it is a well-known value from a specific mathematical sequence.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of log log x as a function for estimating the average number of prime factors, with some supporting its use and others questioning its accuracy based on empirical results. The discussion remains unresolved regarding the best approach to estimate the average number of prime factors.

Contextual Notes

Participants mention the need for a second-order term to correct for small numbers, indicating that the current approach may have limitations in precision for smaller values of x.

soandos
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Is there a function f(x) that will give the average number of prime factors for x_1 0<x_1<x, in a way similar to the way that Li(x)/x gives the approximate odds that a number from 0 to x is prime?
 
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log log x.
 
I tried that for x = 1000, 10,000, 100,000, and it did not work for any of them.
I got the number of factors for 1000 to be 2.87 on average, 3.19 for 10,000, and 3.43 for 100,000
Did I do something wrong?
 
There's a constant factor which depends on what you mean by "prime factor". From your numbers I take it you're counting indistinct prime factors, in which case the constant is 1.03465388...

It predicts an average of (2.97, 3.25, 3.48) versus your calculated (2.87, 3.19, 3.43). It will get more accurate as the numbers involved increase. For example, I calculated http://www.research.att.com/~njas/sequences/A071811 (9) = 4044220058, which compares favorably to the predicted 4065910904.

It should be possible to work out a second-order term (which would be negative) to correct for the presence of small numbers, if you care about that kind of precision.
 
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where would the constant go?
 
soandos said:
where would the constant go?

Expected number of prime factors per number up to x = 1.03465388... + log log x.
 
how did you arrive at this constant?
 
soandos said:
how did you arrive at this constant?

I didn't just derive it: the constant is well-known. It's B2, Sloane's http://www.research.att.com/~njas/sequences/A083342 .
 
Last edited by a moderator:

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