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Hello BP.

Why abs([[-2,1+i,-4]]) returns 4.69041575982 // hp prime sqrt(22)

abs(list[-2,1+i,-4]) returns list[2,sqrt(2),4] // ok
abs([-2,1+i,-4]) returns list[2,sqrt(2),4] // ok


L1 := [ 8, -3.5, 3+4*i, √(-1) ] => [24.0,-10.5,9.0+12.0*i,3.0*i] why it applies approx calculation, to values that are not decimal numbers?

abs on matrices in Xcas probably does a numeric algorithm.

L1 := [ 8, -3.5, 3+4*i, √(-1) ] does not return what you say.