16 May '17 16:551 edit

So one calculator I do i^2 = -1 ok. I do i^3 and it goes 'math error'. But another one (Calctastic app for android) it goes i^3= -i. i^4=1, i^5=i. Is that correct mathematically speaking?

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Serengeti17 May '17 03:09

That is all correct.*Originally posted by sonhouse***So one calculator I do i^2 = -1 ok. I do i^3 and it goes 'math error'. But another one (Calctastic app for android) it goes i^3= -i. i^4=1, i^5=i. Is that correct mathematically speaking?**

Multipling by i on the complex plane is equivalent a rotation of -90degrees.- Joined
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slatington, pa, usa17 May '17 14:042 edits

My cell app gives me i^7 = -i. and i^8=1. Are those crazy numbers? i^9=i according to*Originally posted by wormer***also if you do i^7 or i^8 it will give a crazy number but you must realize that calculators use formulas when calculation this**

CalcTastic app for my android I did i^9.9 and got -0.987688340595 and +0.156434465040i

What about that one?- Joined
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Cape Town17 May '17 14:39

If your calculator accepts complex inputs then the answer should either be correct, should show an error to say it can't do it, or you should throw out your calculator. Once you start questioning its wisdom there is no point having it.*Originally posted by sonhouse***My cell app gives me i^7 = -i. and i^8=1. Are those crazy numbers? i^9=i according to**

CalcTastic app for my android I did i^9.9 and got -0.987688340595 and +0.156434465040i

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slatington, pa, usa17 May '17 14:50

Which is why I am asking. How can you prove that one, i^9.9 = all that stuff?*Originally posted by twhitehead***If your calculator accepts complex inputs then the answer should either be correct, should show an error to say it can't do it, or you should throw out your calculator. Once you start questioning its wisdom there is no point having it.**- Joined
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Cape Town17 May '17 15:291 edit

Either with another calculator ðŸ™‚*Originally posted by sonhouse***Which is why I am asking. How can you prove that one, i^9.9 = all that stuff?**

or log tables.

Have fun.

Actually, I believe its polar coordinates you need:

https://en.wikipedia.org/wiki/De_Moivre%27s_formula#Failure_for_non-integer_powers.2C_and_generalization- Joined
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Cosmopolis17 May '17 21:211 edit

It's the exponential of the log. Suppose x is a real number, and z = i ^ x, then:*Originally posted by sonhouse***Which is why I am asking. How can you prove that one, i^9.9 = all that stuff?**

z = exp(x log(i))

We also have that i = exp(iπ/2), so

z = exp (iπx/2)

But this is only the principal value, the argument of the exponential can have any integer factor of 2πi added to it, so the full expression is:

i^x = exp(iπx(2n + 1/2))

where n is any integer.- Joined
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slatington, pa, usa17 May '17 21:362 edits

Another anamoly in calculators: Doing non-integer factorials.*Originally posted by wormer***graphing calculators will give estimations**

I did 4.5! and it pumped out 59 and change.

But 4.5! = 4.5*3.5*2.5*1.5*0.5=29.53125

But by that same logic, 4.1*3.1*2.1*1.1*0.1 = 2.93601 Which also is odd. What am I doing wrong?

That seemed odd. On my Casio FX-115ES it goes 'math error' even though you can crank through it manually and come out with 29.53125.- Joined
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Cosmopolis17 May '17 21:47

They use something called the Gamma function, which is identical to factorial for the integers but defined for the set of real numbers.*Originally posted by sonhouse***Another anamoly in calculators: Doing non-integer factorials.**

I did 4.5! and it pumped out 59 and change.

But 4.5! = 4.5*3.5*2.5*1.5*0.5=29.53125

But by that same logic, 4.1*3.1*2.1*1.1*0.1 = 2.93601 Which also is odd. What am I doing wrong?

That seemed odd. On my Casio FX-115ES it goes 'math error' even though you can crank through it manually and come out with 29.53125.

Gamma(n) = (n - 1)!

Gamma(1/2) = sqrt(π/2)

The second value is from memory and I might have it wrong, try putting 3/2 into your calculator, press the factorial button and then square the result.

It's defined by an integral and discussed at length in its own Wikipedia page.- Joined
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slatington, pa, usa17 May '17 23:32

So if I input 5.5 it should give me 4.5! right? But it goes 287.88 and change. I wish I knew just what it was computing. If I do 5! it gives 120.*Originally posted by DeepThought***They use something called the Gamma function, which is identical to factorial for the integers but defined for the set of real numbers.**

Gamma(n) = (n - 1)!

Gamma(1/2) = sqrt(π/2)

The second value is from memory and I might have it wrong, try putting 3/2 into your calculator, press the factorial button and then square the result.

It's defined by an integral and discussed at length in its own Wikipedia page.- Joined
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Cape Town18 May '17 10:45

No, that rule only applies for positive integers.*Originally posted by sonhouse***So if I input 5.5 it should give me 4.5! right?**

Imagine you put all the integer factorials on a graph then connected them up with a smooth line.

You would expect 4.5! to be between 5! (120) and 4! (24) but without graphing it, you won't know exactly where. Its not as simple as your subtract 1 calculation.

https://en.wikipedia.org/wiki/Gamma_function- Joined
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Germany18 May '17 15:41

You know that (-1)^2 = 1, (-1)^3 = -1, (-1)^4 = 1, and so on. But you also know that i^2 = -1. Hence i^2 = -1, i^4 = 1, i^6 = -1, and so on. Now take i^2 = -1 and multiply both sides by i, which gives i^3 = -i. Multiply both sides again by i^2 to get i^5 = -i^3 = i. And so on and so forth, giving all the integer powers of i needing to know only that i^2 = -1.*Originally posted by sonhouse***So one calculator I do i^2 = -1 ok. I do i^3 and it goes 'math error'. But another one (Calctastic app for android) it goes i^3= -i. i^4=1, i^5=i. Is that correct mathematically speaking?**

It all becomes pretty intuitive when you learn how complex polar coordinates work.