Discretizing Laplace operator

[LasNikas]

Assuming incompressibility of the fluid, the viscous acceleration simplifies to
$$\frac{d\textbf{v}}{dt} = \frac{\eta}{\rho} \nabla^2 \textbf{v}$$

In Price 2012 the second derivative of a vector is given as

where

Why is Adami 2012 discretizing the above viscous acceleration this way:

Is this the same? Where is the factor of 2?

[svchb]

Another formulation can also be found here:
J. Morris et al., "Modeling Low Reynolds Number Incompressible Flows Using SPH",
In: Journal of Computational Physics, Volume 136, Issue 1, 1997, Pages 214-226.
doi: doi.org/10.1006/jcph.1997.5776

which is also used here:
G. Fourtakas et al., "Local uniform stencil (LUST) boundary condition for arbitrary
3-D boundaries in parallel smoothed particle hydrodynamics (SPH) models",
In: Computers & Fluids, 2019.
doi: doi.org/10.1016/j.compfluid.2019.06.009