[seqfan] Re: Needed: smallest number k with sigma(sigma(k)) = 5k
Lars Blomberg
larsl.blomberg at comhem.se
Sun Jul 3 07:03:20 CEST 2016
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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