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third_party/eigen/doc/examples/TutorialLinAlgComputeTwice.cpp

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+#include <iostream>
+#include <Eigen/Dense>
+
+int main() {
+  Eigen::Matrix2f A, b;
+  Eigen::LLT<Eigen::Matrix2f> llt;
+  A << 2, -1, -1, 3;
+  b << 1, 2, 3, 1;
+  std::cout << "Here is the matrix A:\n" << A << std::endl;
+  std::cout << "Here is the right hand side b:\n" << b << std::endl;
+  std::cout << "Computing LLT decomposition..." << std::endl;
+  llt.compute(A);
+  std::cout << "The solution is:\n" << llt.solve(b) << std::endl;
+  A(1, 1)++;
+  std::cout << "The matrix A is now:\n" << A << std::endl;
+  std::cout << "Computing LLT decomposition..." << std::endl;
+  llt.compute(A);
+  std::cout << "The solution is now:\n" << llt.solve(b) << std::endl;
+}