In the last years, the number of studies on fractional partial differential equations have increased since they can be used in many fields such as physics, engineering, biology and chemistry [1, 2, 3]. Most of these studies focused on obtaining the exact solutions of fractional partial differential equations [13, 14]. But, some of the fractional derivative definitions such as Riemann-Liouville and Caputo do not have capabilities to achieve the exact solutions. Because they do not satisfy some main principles of classical integer order derivative. It is not possible to solve some fractional derivatives by using these definitions. For example
- Riemann-Liouville derivative definition does not satisfy Dαc = 0 where c is real constant and α is not a natural number. (This property satisfies for Caputo derivative definition)
- Riemann-Liouville and Caputo derivatives do not satisfy the derivative of the product of two functions.
Caputo and Riemann-Liouville derivatives do not satisfy the derivative of the quotient of two functions.
Caputo and Riemann-Liouville derivatives do not satisfy the well-known chain rule.
- Caputo and Riemann-Liouville derivatives do not satisfy Dα Dβ (f) = Dα+β (f) usually.
- Caputo derivative definition accepts that the function f is differentiable.
Although the concept of fractional derivative appeared in the middle of the 17th century, in recent years, interest in this subject has increased. The reason is that physical systems can be expressed clearer by fractional derivative. When the literature is examined, we see that a lot of studies have been carried out on fractional derivatives. Different definitions of fractional derivative have been made since 1730s. Because of the limitations of the popular fractional derivative definitions, the scientists worked to find a new definition for fractional derivative which can satisfy all the main principles. Recently, by Khalil et al. introduced the conformable fractional derivative which is a simple, understandable and efficient fractional derivative definition  .
For all t > 0 and α ∈ (0, 1), an α-th order “conformable fractional derivative” of a function is defined by  as
If f is α differentiable in some (0, a), a > 0 and
In the following theorem [1.1], the properties of this new definition are given.
Let α ∈ (0, 1], t > 0 and f, g be α− differentiable functions. Then
- Tα(cf + dg) = cTα(f) + dTα(g), for all a, b ∈.
- Tα(tp) = ptp−α for all p.
- Tα(λ) = 0 for all constant functions f(t) = λ.
- Tα(fg) = fTα(g) + gTα(f).
- If f is differentiable function, then
The conformable fractional derivative has been used to provide new solutions for existing differential equations by many scientists. For example, Ilie et al.  studied general solutions of Riccati and Bernoulli fractional differential equations with conformable fractional derivative. Ta¸sbozan and Kurt  obtained new travelling wave solutions of time-space fractional Liouville and Sine-Gordon equations using conformable fractional derivative definition. Many references can be seen in the literature [15, 16, 17] that used conformable fractional derivative to build the mathematical model of a natural event.
2 The Sub-Equation Method
In this section, we mention a biref description of fractional sub-equation method  . We assume the nonlinear fractional partial differential equation
- Step 1:Using the wave transformation , we get the following equalities.
where k, w are constants to be examined later and. Equation (2.1) can be rewritten in the form of the following ODE by using chain rule  :where prime indicates the known derivative with respect to ξ.
- Step 2.
Assume that Equation (2.3) has one solution in the following form
where ai (0 ≤ i ≤ N) are constants to be determined. N represents a positive integer which is going to found using balancing procedure  in Eq. (2.3) and φ(ξ) satisfies the ordinary differential equation belowwhere σ is a constant. For the Eq. (2.5) , some special solutions are given in the following formulas.
- Step 3. Eqs. (2.4) and (2.5) are substituted into Eq. (2.3) and the coefficients of φi(ξ) are set to zero. This procedure gives a nonlinear algebraic system in ai (i = 0, 1,...,N).
- Step 4.Finally, solving the obtained non-linear algebraic equations system gives us the values of unknown constants. Substituting obtained constants from the nonlinear algebraic system and by help of the formulas (2.6) the solutions of Eq. (2.5) into Eq. (2.4) . This provide the exact solutions for Eq. (2.1) .
3 Implementation of the Sub-Equation Method
3.1 Solution of (1+1) Dimensional Time Fractional KdV6 Equation
Firstly, we consider (1 + 1) KdV6 equation
3.2 Solution of (2+1) Dimensional KdV6 Equation
Consider the time fractional (2+1) KdV6 equation as follows:
In study the sub equation method is implemented to get the new traveling wave solutions of time fractional (1+1) and (2+1) dimensional KdV6 equation successfully. The obtained results indicate that the sub equation method is an efficient, reliable and applicable technique for obtaining the exact solutions of fractional derivatives in conformable sense.
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