Jacobsthal = DifferenceRoot[ Function[{y, n}, {y[n] == y[n - 1] + 2 y[n - 2], y[0] == 0, y[1] == 1}]];
Sum[Jacobsthal[2 i]^2, {i, 0, m - 1}]
Sum[Jacobsthal[2 i + 1]^2, {i, 0, m - 1}]
FullSimplify[ Jacobsthal[2 k + 1]^2 + Jacobsthal[2 k + 3]^2 == 10 + 8 Jacobsthal[2 k] (34 Jacobsthal[2 k - 2] + 15), k \[Element] Integers]
FullSimplify[Jacobsthal[2 k + 1] == 8 Jacobsthal[2 k - 2] + 3, k \[Element] Integers]