# Perturbed n-sphere theory

1. Jul 12, 2011

### andert

Is anyone familiar with books or papers on perturbation theory for closed levels sets in which the equation for the n-sphere is perturbed? For example, the level set:

$$\sum_{i=1}^n x_i^2 + \epsilon p(x_i) = C$$

where $$\epsilon$$ is a small parameter and $$p(x_i)$$ is a positive polynomial such as $$x_i^4$$.

2. Jul 12, 2011

### andert

In particular, say $$A(C,\epsilon)$$ is the "surface area". Then we can expand it:

$$A(C,\epsilon) = A(C,0) + \epsilon (dA/d\epsilon)(C,0) + \dots$$

How do I figure out what $$(d^nA/d\epsilon^n)(C,0)$$ is from the equation for the level set?