[seqfan] Re: Composites in the i*j multplication table A333996

Marc LeBrun mlb at well.com
Fri Oct 2 18:52:28 CEST 2020


I agree, seems confused: A333996 & A334454 currently have identical names,
"Number of distinct composite numbers in the n X n multiplication table.",
yet different values.  And shouldn't the offsets be 0 (not 1) in any case?

Moreover, shouldn't it start 0, 1, 3, 6, 9,...?  From the cardinalities of
{}
{1}
{1,2,4}
{1,2,4,3,6,9}
{1,2,4,3,6,9,8,12,16}
...


> On Oct 2, 2020, at 3:51 AM, Richard J. Mathar <mathar at mpia-hd.mpg.de> wrote:
> 
> Does someone understand the a(4)=7 in the A333996(4) count
> for composites in the 4x4 multiplication table?
> I get a(4)=6, that is the cardinality of {4,6,8,9,12,16},
> and a sequence like 0, 1, 3, 6, 10, 14, 20, 25, 31, 37...
> See also A027424 (cardinality of the set of all i*j) and A000720
> (number of primes in the multiplication table).
> 
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