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Is there an easy way to carry out multivariate power series reversion in Sage?

asked 2025-07-27 23:22:43 +0200

A.S. gravatar image

I have an automorphism $f$ of a power series ring $\mathbb{Z}[[x_1, \dots ,x_n]]$ specified by the n-tuple of power series $f(x_1), f(x_2), \dots , f(x_n)$. I would like to calculate the inverse of this automorphism. This is the multivariate case of power series reversion. Univariate power series reversion is implemented in Sage, but to my knowledge, the multivariate case is not implemented. The answer to this question from 7 years ago indicates that multivariate power series reversion was not yet implemented in Sage at that time.

As far as I know, my options are to either implement the multivariate version of Lagrange inversion in Sage, or try to find some other computer algebra software that might already have this feature. I hope I am wrong, though. Are there any plans to add multivariate power series reversion to Sage? Is there any test code somewhere that might already do this?

Thanks!

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answered 2025-07-29 08:47:21 +0200

FrédéricC gravatar image

Like this

sage: R = LazyPowerSeriesRing(QQ, 'x,y')
sage: x, y = R.gens()
sage: F = R.undefined(valuation=1)
sage: G = R.undefined(valuation=1)
sage: F.define(x / (1 + G)**2)
sage: G.define(y / (1 + F)**3)
sage: F[:3]
[x, -2*x*y]
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A variation on this idea worked for me. Thanks!

A.S. gravatar imageA.S. ( 2025-07-31 11:25:47 +0200 )edit

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Asked: 2025-07-27 23:22:43 +0200

Seen: 3,519 times

Last updated: Jul 29