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| Thank you so much for your answer. I tried to do another thing: First: To substitute by `x'[t]` and `x''[t]` in `ode2` in terms of `ode1` : x'[t] := Sqrt[2 (a''[t]/a[t] + a'[t]/a[t]^2)] x''[t] := D[x'[t], t] Then ... |
| I try to write MA code to define a new modified Einstein tensor: $ G'_{\mu\nu} = G_{\mu\nu}- \frac{1}{3} \left[ \left( \frac{1}{4} R^2 + \nabla_\sigma \nabla^\sigma R \right) g_{\mu\nu} + R R_{\mu\nu} - \nabla_\mu \nabla_\nu R \right], $ ... |
| Hello, I try to solve this system of 2nd order differential equations in variables $x(t,k),y(t,k)$ and $z(t,k)$ $k^2y + 3 H \dot{y} = \frac{1}{2z_0} ( H \dot{z} - \dot{z} ) + \frac{\dot{z}_0}{2z_0} ( 2 H z + \dot{z} ), ~~~~~~~~ (1)$ $H x... |
| Thank you for answer. |
| Dear Wolfram team: I try to calculate : $T_{ab} = g_{ab} F_{gd} F^{gd} - F_a^g F_{bg}$, where $F_{ab} = \partial_a A_b - \partial_b A_a$, So I define $F_{ab}$ by: s[a_, b_] := D[v[b], a] - D[v[a], b] But how to define... |
| Thanks a lot I will give it a try. |