[seqfan] Help me understand these sequences

Charles Greathouse charles.greathouse at case.edu
Mon Mar 29 14:59:06 CEST 2010

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

A144565 ("a(n)=p-th digit of 2^(1/2) where p = n-th prime."):
By the description, this is a duplicate of A090204.  But the values
don't match at all.  What is this?

Charles Greathouse
Case Western Reserve University

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