Section 6.2: Next to that red bar on page 296
Given valid kernels latex($k_{1}(\mathbf{x},\mathbf{x^p}) $) and latex($k_{2}(\mathbf{x},\mathbf{x^p}) $) the following kernels are also valid:
latex($ ck_{1}(\mathbf{x},\mathbf{x^p}) $)
latex($ f(\mathbf{x})k_{1}(\mathbf{x},\mathbf{x^p})f(\mathbf{x^p}) $)
latex($ q(k_{1}(\mathbf{x},\mathbf{x^p})) $)
latex($ exp(k_{1}(\mathbf{x},\mathbf{x^p})) $)
latex($ k_{1}(\mathbf{x},\mathbf{x^p}) + k_{2}(\mathbf{x},\mathbf{x^p} $)
latex($ k_{1}(\mathbf{x},\mathbf{x^p}) * k_{2}(\mathbf{x},\mathbf{x^p}) $)
latex($k_{3}(\mathbf{\Phi(x)^T, \Phi(z)}) $)
latex($ \mathbf{x}^T \mathbf{A} \mathbf{x^p}) $)
latex($ k_{a}(\mathbf{x_{a}},\mathbf{x_{a}^p}) + k_{b}(\mathbf{x_{b}},\mathbf{x_{b}^p}) $)
latex($ k_{a}(\mathbf{x_{a}},\mathbf{x_{a}^p}) * k_{b}(\mathbf{x_{b}},\mathbf{x_{b}^p}) $)