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References to '$\sigma$ function'
1. abundant number by CompositeFan
2. aliquot sequence by PrimeFan
3. almost perfect number by PrimeFan
4. colossally abundant number by CompositeFan
5. deficient number by PrimeFan
6. doubly even number by 1and2and4
7. highly abundant number by CompositeFan
8. hyperperfect number by CompositeFan
9. iterated sum of divisors function by PrimeFan
10. multiplicative function by Wkbj79
11. multiply perfect number by CompositeFan
12. perfect number by Wkbj79
13. power of two by 1and2and4
14. proof of the correspondence between even 2-superperfect numbers and Mersenne primes by PrimeFan
15. singly even number by CompositeFan
16. superperfect number by CompositeFan
17. Wac\l{}aw Sierpi\'nski by PrimeFan