Annual Appeal: Please make a donation (tax deductible in USA) to keep the OEIS running. Over 4500 articles have referenced us, often saying "we would not have discovered this result without the OEIS".
A083531
First difference sequence of A002191. Differences between possible values for sum of divisors of n.
8 and 12 are the 6th and 7th possible values for sigma[x], since they are sum of divisors of x=7 and x=11 respectively, while 9,10,11 are impossible ones so 12-8=4=a(6)= A002191(7)-A002191(6).