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Homomorphisms lifted from base ring in PowerSeriesRing do not preserve precision

asked 2019-01-10 19:06:35 -0500

liu.henry.hl gravatar image

Hi all,

Homomorphisms which are lifted from the base ring seem to be unaware that precision exists in power/Laurent series rings. For example:

sage: R.<x> = PowerSeriesRing(ZZ)
sage: f = Hom(ZZ, ZZ)([1])
sage: Rf = Hom(R, R)(f); Rf
Ring endomorphism of Power Series Ring in x over Integer Ring
  Defn: Induced from base ring by
        Ring endomorphism of Integer Ring
          Defn: 1 |--> 1
sage: Rf(1 + x + O(x^2))
1 + x

Can someone confirm that the expected output should be 1 + x + O(x^2), and that this is a bug?

Thanks,

Henry

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answered 2019-01-23 00:44:20 -0500

vdelecroix gravatar image

Indeed, this is broken. Note that it works fine with the identity morphism

sage: fR = Hom(R, R).one()
sage: fR
Identity endomorphism of Power Series Ring in x over Integer Ring
sage: fR(1 + x + O(x^2))
1 + x + O(x^2)

But not for the lifted identity

sage: fZZ = Hom(ZZ, ZZ).one()
sage: fR2 = Hom(R, R)(fZZ)
sage: fR2(1 + x + O(x^2))
1 + x
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Asked: 2019-01-10 19:06:35 -0500

Seen: 56 times

Last updated: Jan 23