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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).