Tag: calculation

  • Changing regularization, II

    Today I went back to trying to understand the solution when using the original regularization. While doing so it occurred to me that if I use a slightly different regularization, I can get a closed-form solution for the feedforward connectivity $Z$, and without most (though not all) of the problems I was having in my…

  • Wrangling quartics, III

    We are trying to understand the connectivity solutions $Z$ found when minimizing the objective $$ {1 \over 2 n^2 } \|X^T Z^T Z X – C\|_F^2 + {\la \over 2 m^2}\|Z – I\|_F^2.$$ Recap We found in the previous post that solutions satisfy$$ {1 \over \la’} \left(S^2 \wt Z_{UU}^2 S^2 – S \wt C_{VV} S…

  • Wrangling quartics, II

    In the last post on this topic, we saw that when optimizing the objective$$ {1 \over 2 n^2 } \|X^T Z^T Z X – C\|_F^2 + {\la \over 2 m^2}\|Z – I\|_F^2,$$ any solution $Z$ is symmetric and satisfies $${2 \over n^2} \left( XX^T Z^2 XX^T – X C X^T\right) + {\la \over m^2} I…

  • Inverting arrowhead matrices.

    I need to invert a matrix of the form $$ M = I + S^2 H S^2,$$ where $H$ is a symmetric matrix, and $S^2$ is diagonal. The elements of $S^2$ drop off very quickly, so what remains of $H$ are its first column and first row, scaled by $S_{1}^2 S^2$. The result is that…

  • How many lateral dendrites cross a granule cell arbor?

    The projection neurons of the olfactory bulb are the mitral cells and tufted cells. Most mitral cells don’t communicate with each other directly. Instead, they interact through the synapses that their lateral dendrites make onto granule cell abors. Activation of these synapses excites the target granule cells, which in turn inhibit the mitral cells that…