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Old 2017-09-14, 04:52   #1
sean's Avatar
Aug 2004
New Zealand

22210 Posts
Default Iteration of (sigma(n)+phi(n))/2

In recent weeks there has been some interest on the seqfan mailing list about iterating the map (sigma(n)+phi(n))/2 (see for example A291790).

In many cases the map results in a fraction (i.e. odd/2) and the iteration finishes. However, there appears to be cases (perhaps a lot of cases for large n) where the iteration appears unbounded and continues indefinitely (not unlike certain aliquot sequences or home primes, etc.). It is not clear why this should be.

The smallest unresolved case is starting with n=270, which has now survived 515 iterations of the map. The unresolved cases for n<1000 are 270, 440, 496, 702, 737, 813, 828, 897, and 905. There are other values less than 1000 unresolved, but their trajectories converge with one of the nine listed. All of them have survived at least 400 iterations.

All the factorizations for the existing steps have been added to I'm not planning on taking these further myself, but there is plenty of scope for fairly easy factorization here. Some the composites are still down in 100 digit range and even the hardest (for the 270 trajectory) is only a C138.
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