[seqfan] Re: Needed: smallest number k with sigma(sigma(k)) = 5k

[Lars Blomberg]

Hello,

I am trying to extend the limit for a(5), but progress is slow.

/Lars

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Från: SeqFan [mailto:seqfan-bounces at list.seqfan.eu] För Neil Sloane
Skickat: den 2 juli 2016 06:28
Till: Sequence Fanatics Discussion list <seqfan at list.seqfan.eu>
Ämne: [seqfan] Needed: smallest number k with sigma(sigma(k)) = 5k

a(n) = smallest k with sigma(sigma(k)) = n*k (or 0 if no such k exists)  is A272930

and has a gaping hole in it:

1, 2, 8, 15,  ?, 42, 24, 60, 168, 480, 4404480, 2200380, 57120, 217728, 1058148, 7526400, ?, ...


The comments say the search for a(5) has gone out to 5*10^11, and that it is conjectured to be nonzero.

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