Comparing even powers of $i$ with 1

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Comparing even powers of

$i$ with 1

Ed Scheinerman
61 ●3 ●6 ●11

Computing $i^4$ yields $1$ , but it's not the same $1$ as when I type $1$ . Compare these two evaluations:

sage: I^4 == 1
1 == 1
sage: I^2 == 1
-1 == 1

I would have expected the first to yield True and the second to yield False. Is Sage's answer a desirable default behavior?

I see that the first $1$ is a sage.symbolic.expression.Expression and the other $1$ is a sage.rings.integer.Integer (likewise for the $-1$ ). How can I make the comparison evaluate as one might reasonably expect mathematically?

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answered 11 years ago

tmonteil
27483 ●32 ●205 ●519 http://wiki.sagemath.o...

updated 11 years ago

Since I^4 is a symblic expression, I^4 == 1 is also a symbolic expression:

sage: expr = (I^4 == 1)       
sage: expr.parent()
Symbolic Ring

You can evaluate it by typing:

sage: bool(I^4 == 1)
True

But warning: if the expression is complicated and maxima is not able to decide wether it is True or False, then bool(expr) will return False.

A good advice is to avoid the symbolic ring as much as possible. For example, you can work in the algebraic field:

sage: QQbar(I)^4 == 1
True

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Asked: 11 years ago

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Last updated: Oct 15 '13

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