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Difference between Riesz derivative and fractional Laplacian on the proper subset of ℝ

  • Caiyu Jiao , Abdul Khaliq , Changpin Li EMAIL logo and Hexiang Wang
Published/Copyright: November 22, 2021

Abstract

In general, the Riesz derivative and the fractional Laplacian are equivalent on ℝ. But they generally are not equivalent with each other on any proper subset of ℝ. In this paper, we focus on the difference between them on the proper subset of ℝ.

Acknowledgements

The work was partially supported by the National Natural Science Foundation of China under Grant nos. 11926319 and 11926336.

References

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6 Appendix

Theorem 6.1

If f(x) ∈ ACma) ∩ L1a) then RLD,xαf(x), (m − 1 ≤ α < m, mZ+) exists almost everywhere on Ωa.

Proof

For simplicity, we first prove the case of α ∈ (0, 1) for the left-sided Riemann-Liouville derivative. For arbitrary c1 > 0, we have

RLD,xαf(x)=1Γ(1α)ddxxc1x+xc1f(y)(xy)αdy. (6.1)

Since f(y) ∈ BVa), there exists two bounded monotone increasing functions g1(y) and g2(y) on Ωa such that

f(y)=g1(y)g2(y). (6.2)

Then we have

xc1xf(y)(xy)αdy=0c1f(xy)yαdy=0c1g1(xy)yαdy0c1g2(xy)yαdy:=G1(x)G2(x). (6.3)

It is obvious that G1(x) and G2(x) are bounded functions on Ωa. For arbitrary x1, x2 ∈ Ωa satisfying x1 < x2, it holds that

G1(x2)G1(x1)=0c1g1(x2y)g1(x1y)yαdy0. (6.4)

With similar way for G2(x), we can derive both G1(x) and G2(x) are bounded monotone increasing functions. Thus, we have

F(x)=xc1xf(y)(xy)αdyBV(Ωa), (6.5)

so that F(x) is differentiable almost everywhere on Ωa. For the second part, we have

ddxxc1f(y)(xy)αdy=limΔx01Δxx+Δxc1f(y)(x+Δxy)αdyxc1f(y)(xy)αdy=limΔx01Δxxc1x+Δxc1f(y)(x+Δxy)αdyαxc1f(y)(xy)α+1dy=f(xc1)limΔx01Δxxc1x+Δxc1dy(x+Δxy)ααxc1f(y)(xy)α+1dy=f(xc1)limΔx011α(c1+Δx)1α(c1)1αΔxαxc1f(y)(xy)α+1dy=f(xc1)(c1)ααxc1f(y)(xy)α+1dy, (6.6)

where integral mean value theorem and dominated convergence theorem are utilized. Combining (6.5) and (6.6), we obtain RLD,xαf(x) exists almost everywhere on Ωa for α ∈ (0, 1).

When α ∈ (m − 1, m), since f(x) ∈ ACma), f(x) has continuous derivative up to m − 1 on closed real line ℝ and f(m−1)(x) ∈ ACa). Then, we obtain

dm1dxm1xc1xf(y)(xy)αm+1dy=xc1xfm1(y)(xy)αm+1dyBV(Ωa), (6.7)

where c1 is a positive constant. Thus, 1Γ(mα)dmdxmxc1xf(y)(xy)αm+1dy exists almost everywhere on Ωa. Also, we have

dmdxmxc1f(y)(xy)αm+1dy=dm1dxm1f(xc1)(c1)αm+1(αm+1)xc1f(y)(xy)αm+2dy=dm2dxm2[f(xc1)(c1)αm+1(αm+1)f(xc1)(c1)αm+2+(αm+1)(αm+2)xc1f(y)(xy)αm+3dy]=k=0m1Γ(mα)Γ(kα+1)f(k)(xc1)(c1)αk+Γ(mα)Γ(α)xc1f(y)(xy)α+1dy. (6.8)

Combining equations (6.7) and (6.8), we get

RLD,xαf(x)=dmdxmRLD,x(mα)f(x)=1Γ(mα){dmdxmxc1xf(y)(xy)α+1mdy+k=0m1Γ(mα)Γ(kα+1)f(k)(xc1)(c1)αk+Γ(mα)Γ(α)xc1f(y)(xy)α+1dy}. (6.9)

By means of the condition of f(x),RLD,xαf(x) exists almost everywhere on Ωa. For the other cases, we can derive the result similarly. The proof is completed. □

Received: 2021-05-21
Revised: 2021-10-14
Published Online: 2021-11-22
Published in Print: 2021-12-20

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