A correction (Exam 2015)

A correction in the solution of the exam from 2015; thanks to Kristine for pointing this out: In 1c) when one inserts u(x,t)=X(x)T(t) into the eq., one gets T'/T=X''/(X-X'')=lambda. This gives X''=(lambda/(1+lambda))X, from which one gets the solution u as described in the exam solutions.