HP Prime: Unexpected "poly1" after cross product
03-03-2022, 10:57 PM
Post: #1
 RickyFlips Junior Member Posts: 8 Joined: Jul 2020
HP Prime: Unexpected "poly1" after cross product
What does it mean when HP Prime prepends "poly1" to the vector/matrix output of a calculation?

I calculated the following:

CROSS([-0.01 0 0],[60 0 0]) - 0.02^2 * 60 + CROSS([0 0 -0.02],[0.5 0 0])

Which returns just the following, as expected: [-0.62 -0.024]

However, when I copy this output to the entry field (I forget what HP calls this) to do a second calculation, my HP Prime prepends "poly1," to it, so I get:

poly1[-0.62 -0.024]

I haven't been able to figure out what this means or why it shows up, only that I have to manually delete it before doing the next calculation to get the expected answer.

Thank you!
03-04-2022, 07:18 AM
Post: #2
 parisse Senior Member Posts: 1,322 Joined: Dec 2013
RE: HP Prime: Unexpected "poly1" after cross product
poly1[ is a delimiter for a list representing a univariate polynomial (with coefficients in decreasing order). If you add lists that have different sizes, then they can not be considered as vectors (at least in the same vector space), and addition will mean univariate polynomials addition.
Using poly1[ instead of [ as a list left delimiter is a way to disambiguate some operations like *, if you multiply [1,2]*[3,4] by default dot product is used and it returns a real, if you multiply poly1[1,2]*[3,4] polynomial multiplication will be used.
03-04-2022, 07:05 PM
Post: #3
 RickyFlips Junior Member Posts: 8 Joined: Jul 2020
RE: HP Prime: Unexpected "poly1" after cross product
@parisse ~ Thank you! That makes perfect sense, and I'd wondered if adding vectors and non-vectors might cause the Prime to hiccup. I'll just use the vector form of that term in future calculations.
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