# [seqfan] Re: Help me understand these sequences

Ray Chandler rayjchandler at sbcglobal.net
Mon Mar 29 16:38:17 CEST 2010

```Probably not, but here's my best guess.

The sequence referenced in A171994 should be A171697 rather than the non-existent A171897.

I think A171994 is intended to be the subset of A171697 that have three (not free) prime factors.

If so, then corrections to terms follow:
435 should be 425
insert 1519
1775 should be 1775

and the corrected sequence becomes

245, 325, 343, 425, 475, 637, 665, 715, 805, 833, 845, 847, 925,
1001, 1025, 1045, 1075, 1175, 1265, 1331, 1463, 1475, 1505, 1519,
1645, 1675, 1705, 1729, 1771, 1805, 1855, 1885, 1955, 2023, 2035,
2057, 2075, 2093, 2107, 2185, 2197, 2225, 2233, 2255, 2261, 2303,
2365, 2405, 2431, 2485...

Typical of the quality of submissions from this author.

Guess whether the definition is wrong, the terms are wrong or references are wrong.  Answer - all of the above.
Ray

> Is it worth your (and our) time and efforts if the author
> doesn't respond?
>
>        Klaus Brockhaus
>
>
> Charles Greathouse schrieb:
> > A171994 ("Twin natural nonprimes are products of free primes."):
> > I can see that the members all have three prime factors,
> but I don't
> > know what makes them twin natural nonprimes.  The definitions
> > elsewhere suggest that this means numbers n with n+2 nonprime, but
> > then why are 8 and 63 not in the sequence?  (And others beside: I
> > don't think it's a case of an omission but my
> misunderstanding,)  And
> > the numbers don't have n+2 with 3 prime factors either, so
> that theory
> > also fails.  [I sent a similar message to the sequence
> author with no
> > response.]
> >

```