[seqfan] Re: Is A026477 determined by prime signatures?
Charles Greathouse
charles.greathouse at case.edu
Fri Aug 26 20:55:20 CEST 2016
I also haven't found a good way of discovering which prime signatures are
in the sequence. In principle this is combanatorial but I don't know of a
good algorithm.
Charles Greathouse
Case Western Reserve University
On Fri, Aug 26, 2016 at 7:17 AM, Don Reble <djr at nk.ca> wrote:
> A026477... a(1) = 1, a(2) = 2, a(3) = 3; and for n > 3,
>> a(n) = smallest number > a(n-1) and not of the form a(i)*a(j)*a(k)
>> for 1 <= i < j < k < n.
>>
>> It seems that if two numbers have the same prime signature (multiset of
>> prime exponents) then either both or neither are in the sequence, but I
>> can't prove this.
>>
>
> Just do strong induction on the number of prime factors (sum of
> signature exponents).
>
> ... prime powers p^r can only be r = {1,2,4,8,15,22...}, ...
>>
>
> Yes: A026474.
> Also, square-free numbers have 3n+1 prime factors.
>
> This suggests that A026477 intersect A025487 (least value of each
> signature) would be a worthy sequence. But I don't see how to easily
> recognize those signatures.
>
> value signature
> 1:
> 2: 1
> 4: 2
> 16: 4
> 120: 3 1 1
> 210: 1 1 1 1
> 216: 3 3
> 256: 8
> 384: 7 1
> 2880: 6 2 1
> 6300: 2 2 2 1
> 7200: 5 2 2
> 15360: 10 1 1
> 15552: 6 5
> 26880: 8 1 1 1
> 27648: 10 3
> 32768: 15
> 49152: 14 1
> 73728: 13 2
> 83160: 3 3 1 1 1
> 120120: 3 1 1 1 1 1
> 189000: 3 3 3 1
> 510510: 1 1 1 1 1 1 1
> 921600: 12 2 2
> 1399680: 7 7 1
> 1966080: 17 1 1
> 2365440: 11 1 1 1 1
> 2822400: 8 2 2 2
> 2985984: 12 6
> 3440640: 15 1 1 1
> 4194304: 22
> 4860000: 5 5 4
> 5670000: 4 4 4 1
> 6291456: 21 1
> 6912000: 11 3 3
> 9437184: 20 2
> 10644480: 10 3 1 1 1
> 15375360: 10 1 1 1 1 1
> 60466176: 10 10
> 65345280: 8 1 1 1 1 1 1
> 71663616: 15 7
> 117964800: 19 2 2
> 127401984: 19 5
> 161243136: 13 9
> 251658240: 24 1 1
> 251942400: 9 9 2
> 302776320: 18 1 1 1 1
> 361267200: 15 2 2 2
> 440401920: 22 1 1 1
> 536870912: 29
> 805306368: 28 1
> 892371480: 3 1 1 1 1 1 1 1 1
> 1109908800: 6 2 2 2 2 1
> 1207959552: 27 2
> 1327104000: 17 4 3
> 1968046080: 17 1 1 1 1 1
> 4232632320: 11 10 1 1
> 6469693230: 1 1 1 1 1 1 1 1 1 1
> 9172942848: 22 7
> 9932482560: 11 1 1 1 1 1 1 1
> 10883911680: 12 12 1
> 12570798240: 5 5 1 1 1 1 1 1
> 13759414272: 21 8
> 13783770000: 4 4 4 1 1 1 1
> 15330615300: 2 2 2 2 2 2 1
> 16307453952: 26 5
> 23279477760: 10 10 1 1 1
> 24461180928: 25 6
> 32212254720: 31 1 1
> 32248627200: 16 9 2
> 38755368960: 25 1 1 1 1
> 39729690000: 4 4 4 3 1 1
> 56371445760: 29 1 1 1
> 68719476736: 36
> 103079215104: 35 1
> 114223549440: 10 1 1 1 1 1 1 1 1
> 154618822656: 34 2
> 156728328192: 15 14
> 169869312000: 24 4 3
> 251909898240: 24 1 1 1 1 1
> 408410100000: 5 5 5 5
> 717001084800: 7 2 2 2 2 1 1 1
> 812665405440: 17 11 1 1
> 828120733440: 8 1 1 1 1 1 1 1 1 1
>
> --
> Don Reble djr at nk.ca
>
>
>
> --
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