# [seqfan] Engel Expansion of constants = p-rough numbers, prime p

B.W.J. Irwin bwji2 at cam.ac.uk
Wed Dec 21 18:34:01 CET 2016

```I was prompted by Joerg to ask here: (I've just joined so apologies if
this is the wrong format)

Key points:
Engel Expansion: A unique weakly increasing integer sequence
corresponding to a real constant.
http://mathworld.wolfram.com/EngelExpansion.html

For most well behaved sequences, there seems to be a usually complicated
constant than then contains the information of the sequence.

p-rough numbers: The set of integers not divisible by primes less than
p.
http://mathworld.wolfram.com/RoughNumber.html

For larger p, the sequence gets denser in primes as the small divisors
are sifted out.
In general for p-rough numbers, all the numbers between a(1)=1 to
a(...)=p^2 are prime.

I think I have found a general expression (infinite sum, or finite sum
of hypergeometric functions) for the constant whose Engel expansion is
the p-rough numbers, for any p. The higher terms are just conjectural at
the moment, apart form the 5-rough and 7-rough numbers, but they seem to
work. The constant seems to tend to 1 as p tends to infinity.

https://www.authorea.com/users/5445/articles/144462/_show_article

They get quickly harder to evaluate as p increases, but seem to be
working for the first few.

Mainly pertains to A279664.