Landau Notation: Writing Functions in Terms of Other Functions

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Discussion Overview

The discussion revolves around the interpretation and application of Landau notation, specifically how to express the relationship between two functions, f(x) and g(x), when f(x) is much larger than g(x). Participants explore the appropriate symbols to use, such as \(\Omega\), \(\omega\), \(o\), and \(O\), and clarify the implications of these notations.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that if f(x) >> g(x), it could be expressed as f(x) = o(g(x)), but they are uncertain.
  • Another participant asserts that f(x) >> g(x) translates to f(x) = \Omega(g(x)), while noting that using an equality with Landau symbols may be considered an abuse of notation.
  • A different participant emphasizes that log n << n should be expressed as o(n) rather than O(n), arguing that it indicates log n is much smaller than n and not comparable.
  • Another participant counters that it should be O(n) and references the "Vinogradov symbol" to support their claim.

Areas of Agreement / Disagreement

Participants express differing views on how to represent the relationship between f(x) and g(x) when f(x) is much larger than g(x). There is no consensus on whether to use o or O for this relationship, indicating an unresolved debate.

Contextual Notes

Participants reference specific interpretations of Landau notation and its implications, but there are unresolved aspects regarding the appropriateness of using equalities with Landau symbols and the definitions of the symbols themselves.

flouran
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I have a rather simple question which requires a direct answer:

We have two functions, f(x) and g(x).

I know that f(x) << g(x) is the same as f(x) = O(g(x)).
But if f(x) >> g(x), how can I write f(x) in terms of g(x) using the one of the four Landau symbols (\Omega, \omega, o, or O)?

I suspect that f(x) >> g(x) means the same as f(x) = o(g(x)), but I am not sure.
 
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f(x) >> g(x) translates as:
f(x) = \Omega(g(x)).
Although according to Knuth, using an equality in front of a Landau symbol is supposedly abuse of notation (apparently \in is preferred).
 
Last edited:
flouran said:
I have a rather simple question which requires a direct answer:

We have two functions, f(x) and g(x).

I know that f(x) << g(x) is the same as f(x) = O(g(x)).

Actually, I would say o not O for this.

log n << n means log n is MUCH SMALLER than n, not allowed to be comparable to n.
 
g_edgar said:
Actually, I would say o not O for this.

log n << n means log n is MUCH SMALLER than n, not allowed to be comparable to n.
No, it's O. Look up "Vinogradov symbol" and you'll see that I am right.
 

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