Discussion about math, puzzles, games and fun. Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ π -¹ ² ³ °

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**Hannibal lecter****Member**- Registered: 2016-02-11
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Hello,

is -∞ < 1 ?

Wisdom is a tree which grows in the heart and fruits on the tongue

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**ganesh****Administrator**- Registered: 2005-06-28
- Posts: 25,083

Hi Hannibal lecter,

Yes. -∞ < 1

The list of integers is

-∞, -∞ + 1, -∞ + 2, -∞ + 3, .........., -3, -2, -1, 0, 1, 2, 3, ..........., ∞

It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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**bob bundy****Administrator**- Registered: 2010-06-20
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hi Hannibal lecter

The number line is usually shown running left to right with (say) 2 to the left of 3 etc.

The symbol < (which we read as "less than") actually means "is to the left of", so -4 < -3 etc. As -∞ is the 'leftmost' place you could have on the line* -∞ < x for all x on the line.

* But as you cannot get to this place it is (strictly) not on the line.

Bob

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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You have to be careful what you mean by the statement "". What do the symbols and mean here? is not a real number, but we can treat it like a number by adding it on to our usual collection of real numbers. That is, if is the set of all real numbers, then you can 'add on' infinity and negative infinity by considering the extended real numbers, . Then, not only can you define arithmetic with these infinities (and this has significant geometric implications -- see for instance stereographic projection, which you can read about), but you can even define an ordering relation on the extended real number line so that it makes sense to say something like , for any .

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**Hannibal lecter****Member**- Registered: 2016-02-11
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thanks you so much,

and is 1/-∞ < 1 too?

Wisdom is a tree which grows in the heart and fruits on the tongue

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**bob bundy****Administrator**- Registered: 2010-06-20
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The calculation 1/-∞ doesn't really exist as the rules of arithmetic fail for ∞ .

I suppose you could ask what happens to 1/x as x tends to -∞ .

eg 1/-1000 < 1

1/-1000000000 < 1

etc.

It looks like 1/x tends to zero from below (from the left) as x tends to -∞ and we know that 0 < 1.

So I suppose the answer to your question (ignoring the rules for a moment) is YES.

Bob

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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