# Correction to: A $${\texttt {p}}({\cdot })$$-Poincaré-type inequality for variable exponent Sobolev spaces with zero boundary values in Carnot groups

## Correction to: Anal.Math.Phys. (2018) 8:289–308 https://doi.org/10.1007/s13324-018-0235-7

In this short note, we remark on the main theorem of [1, Theorem 5.1]. First, the authors would like to thank the referee for pointing out that a more general result related to [1, Theorem 5.1], was proved for vector fields satisfying Hörmander’s condition by Li et al. [3, Theorem 1.10]. In [3, Theorem 3.1], the authors first show a more general result for boundedness of fractional integral operators in variable Lebesgue spaces on spaces of homogeneous type and combine this with a representation formula proved in [2] to establish a higher order Poincaré inequality for vector fields satisfying Hörmander’s condition on Boman chains (and therefore metric balls). Here they took the assumptions that $${\texttt {p}}({\cdot })$$ is log-Hölder continuous and $$1< {\texttt {p}}^- \le {\texttt {p}}^+ < Q/m$$, where Q is the homogenous dimension and m is the dimension of the variable exponent space (in our case $$m=1$$).

However, the main theorem [1, Theorem 5.1] does not follow directly from [3, Theorem 1.10]. The difference is in the assumption that $${\texttt {p}}({\cdot })$$ is log-Hölder continuous in [3, Theorem 1.10] while [1, Theorem 5.1] requires that the variable exponent $${\texttt {p}}({\cdot })$$ satisfies the jump condition. This allows for the case where $${\texttt {p}}^- > Q$$ in the gauge balls discussed in Section 5 of [1]. Thus, the condition $$1< {\texttt {p}}^- \le {\texttt {p}}^+ < Q$$ in [3, Theorem 1.10] is violated. Consequently, when $${\texttt {p}}^- > Q$$, then the hypotheses of [3, Theorem 1.10] do not hold but those of [1, Theorem 5.1] do hold.

Furthermore, the main theorem [1, Theorem 5.1] does not follow from combining [3, Theorem 3.1] and the representation formula of functions with boundary value zero for vector fields established in [4]. Again, we have the possibility that $${\texttt {p}}^- > Q$$ in gauge balls but this fails the conditions in [3, Theorem 3.1].

In both cases, it is obvious that if $${\texttt {p}}^+ \le \frac{Q{\texttt {p}}^-}{Q- {\texttt {p}}^-}$$ in gauge balls, then we have the jump condition satisfied and this also agrees with the conditions in [3, Theorem 1.10] and thus [1, Theorem 5.1] does indeed follow directly from [3, Theorem 1.10]. Similarly, this is also true for [3, Theorem 3.1] and we could use representation formulas in [4] to easily obtain [1, Theorem 5.1].

## References

1. 1.

Bieske, T., Freeman, R.D.: A $$p(x)$$-Poincaré-type inequality for variable exponent Sobolev spaces with zero boundary values in Carnot groups. Anal. Math. Phys. 8, 289–308 (2018)

2. 2.

Franchi, B., Lu, G., Wheeden, R.L.: Representation formulas and weighted Poincaré inequalities for Hörmander vector fields. Ann. Inst. Fourier (Grenoble) 45, 577–604 (1995)

3. 3.

Li, X., Lu, G.Z., Tang, H.L.: Poincaré and Sobolev inequalities for vector fields satisfying Hörmander’s condition in variable exponent Sobolev spaces. Acta Math. Sin. Engl. Ser. 31(7), 1067–1085 (2015)

4. 4.

Lu, G.: Weighted Poincaré and Sobolev inequalities for vector fields satisfying Hörmander’s condition and applications. Rev. Mat. Iberoam. 8, 367–439 (1992)

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Correspondence to Thomas Bieske.

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The original article can be found online at https://doi.org/10.1007/s13324-018-0235-7.

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Bieske, T., Freeman, R.D. Correction to: A $${\texttt {p}}({\cdot })$$-Poincaré-type inequality for variable exponent Sobolev spaces with zero boundary values in Carnot groups. Anal.Math.Phys. 9, 1611–1612 (2019). https://doi.org/10.1007/s13324-018-00278-3