Question: how to calculate negative compound interest?
Answer: The formula is n × (1 + growth rate)p. This works because you're basically doing (for an growth of 1% over 2 periods) n × 1.01 × 1.01 = n × 101% × 101%. This also works for negative growth: then it's just n × 0.98 x 0.98, (2% deflation) for example.