The dimensions of the homomorphism spaces to indecomposable modules over   the four subspace algebra
  This is the addendum to the paper "On the Multiplicity Problem and the Isomorphism Problem for the Four Subspace Algebra" Communications in Algebra, 40:6 (2012), 2005-2036 (DOI: 10.1080/00927872.2011.570830). We give here the full proof of Proposition 3.3, describing the formulas for the dimensions of the homomorphism spaces to indecomposable modules over the four subspace algebra \Lambda. The reader can also find here the full description of indecomposable \Lambda-modules. 
Andrzej Mr\'oz
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