3 1 8 D O C U M E N T S 3 3 2 , 3 3 3 D E C E M B E R 1 9 2 8 332. To Julius Mueller-Alberti[1] [Berlin/Gatow?, after 12 December 1928][2] Your plan seems reasonable and welcome, like every effort seeking to counter through positive work the narrow nationalist attitude still prevailing in the school.[3] 333. To Chaim Herman Müntz [Berlin?,] 13 December 1928 Dear Mr. Müntz, The identity for the can be found as follows: One starts with the identity for [the case of] :[1] . By cyclic permutations of k, l, m, one obtains two other equations. Then all three are added. The first term gives, together with the second, once cyclically permuted term: . We combine the third term together with the fourth, twice cyclically permuted: or + Thus, one initially obtains + or + (I) This is still not a genuinely covariant form of the identity, since differentiating with regard to m does not give a “covariant” derivative (we would write those as ). However, we have . (1) If we permute cyclically and add, then on the right, owing to (1),[2] the terms orig- inating with the first two terms on the right in (1) cancel out. If we combine the third with the once cyclically permuted fourth, then we obtain or . curvature 0 i l m – i m l i l m i m l – + + l m –i i l m i l m – l i m i l m – . –. – i ml . . + + 0 i l m + . . + i lm . . + + 0 i kl m i l m i l m l m i i l m – i lm – + i l m i l m – – l m –i