D O C U M E N T 1 5 9 J A N U A R Y 1 9 2 6 1 7 9 (16) , and demand that the antisymmetrical component of , formed in ac- cordance with Rainich, vanishes: (17) . Exactly as above, one proves that this condition is equivalent to the equations (18) , which agrees with (2a), (3). Thus, it is shown that the gravitational field equations (2a) suggested by the cos- mological problem and the structure of the electromagnetic energy tensor allows for a natural mathematical interpretation. 159. From Eduard Einstein Zurich, 10 January 1926 Dear Papa, I see to my horror that, again, I have not written to you for a considerable time.[1] But that’s the way it goes: when people are so far away from each other, they forget mutual obligations in favor of those that are more obvious. And I must say, whereas otherwise I like to write, I really don’t much like letter writing. There is something uncanny about speaking to a person that one doesn’t even see and talking about per- sonal things. Holidays are here again. But they are short, and we stayed in Zurich (total bankruptcy!)[2] and the weather is bad, a sentimental summerlike warmth. So the holidays are not as nice as one might have expected. Sometimes I’m even bored, but that ultimately does no harm it reduces the contrast between school & vacation.[3] And how do I spend my time? I don’t know I don’t usually do much during vacations and especially now during the holidays, time is spent with visits, etc. But I am now playing the piano quite a bit. In my lessons, I am now playing Bach’s “Well-Tempered Clavier”[4] and that is, of course, wonderfully beautiful. But I also have not given up my fondness for the more modern composers, namely my heroes Reger & Debussy.[5] E.g., there are a number of little preludes and fugues by Reger,[6] which in any case have quite a bit of Bach in them, but are com- pletely charming. We now have Haydn’s violin sonatas here,[7] but—no harm meant—I find them truly boring & insignificant. In my opinion, they don’t hold a candle to Mozart’s sonatas. Rik, lm * Rik lm , kEik lm , += Aik lm , * Rik lm , * Aik lm , * 1 2 --( - Rik lm , * R* ik lm , – ) 0 = = Gim * Rim * 1 4 -- - gimR* – 0 = =