function G_apro = STS(G,d,MPP)
%% 失效点处二阶taylor展开
%差分法计算偏导数
deta = 1e-7;%初始化
D_p = diag(deta.*ones(1,d));

for i = 1:d
    Pd(i) = (G(MPP+D_p(i,:))-G(MPP))./deta; 
end
%计算hessian矩阵
Hessian = zeros(d);
for i = 1:d
    for j = 1:d
        Hessian(i,j) = (G(MPP+D_p(i,:)+D_p(j,:))-G(MPP+D_p(i,:)-D_p(j,:)) ...
            -G(MPP-D_p(i,:)+D_p(j,:))+G(MPP-D_p(i,:)-D_p(j,:)))./(4*deta^2);
    end
end
%近似参数计算
a_hat = 1/2*MPP*Hessian*MPP.' - Pd*MPP.';
b_hat = Pd - MPP*Hessian.';
c_hat = 1/2 * Hessian;
%二阶近似函数
G_apro = @(U) a_hat + b_hat*U.' + U*c_hat*U.';