[Scilab-users] slash (/) help page confusing ?

[Stéphane Mottelet]

Le 30/08/2018 à 09:32, philippe a écrit :
> Hi
>
> before scilab6 there was some ambiguity between 1/B=1./B=(1.)/B  since
> 1. = 1 for floating point real numbers. We can see that this problem is
> solved now :
>
>
> --> B=1:3
>   B  =
>
>     1.   2.   3.
>
>
> --> 1./B
>   ans  =
>
>     1.   0.5   0.3333333
>
>
> --> 1/B
>   ans  =
>
>     0.0714286
>     0.1428571
>     0.2142857
>
> but in the help page it is written that "X=A/B is the solution of X*B=A"
> this false for  A=1  and B=1:3  as you can see :
>
>
> --> B*(1/B)  // =1 !!!!
>   ans  =
>
>     1.
>
>
> --> (1/B)*B  //  should be 1 ?
>   ans  =
>
>     0.0714286   0.1428571   0.2142857
>     0.1428571   0.2857143   0.4285714
>     0.2142857   0.4285714   0.6428571
>
> but this is true for general matrix
>
> --> A=[1 2 ; 3 4]
>   A  =
>
>     1.   2.
>     3.   4.
>
>
> --> B=[4 5 ; 6 7]
>   B  =
>
>     4.   5.
>     6.   7.
>
>
> --> B*(A/B) // sould be A ?
>   ans  =
>
>     17.5  -8.5
>     25.5  -12.5
>
>
> --> (A/B)*B // =A
>   ans  =
>
>     1.   2.
>     3.   4.
>
>
> this looks confusing ?
No.

For square A, B, invertible B, "A/B" means "A*inv(B)", hence 
B*(A/B)=B*A*inv(B) cannot be equal to A (matrix product is not commutative)

S.
>
> Best regards,
> Philippe
>
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-- 
Stéphane Mottelet
Ingénieur de recherche
EA 4297 Transformations Intégrées de la Matière Renouvelable
Département Génie des Procédés Industriels
Sorbonne Universités - Université de Technologie de Compiègne
CS 60319, 60203 Compiègne cedex
Tel : +33(0)344234688
http://www.utc.fr/~mottelet