How to develop a function which returns the 'n'th number divisible only by primes (excluding primes)?

Asked Mar 19 '16 at 05:46

Active Mar 19 '16 at 08:13

Viewed 40 times

To elaborate, I'd like to programmatically find numbers which can only be divisible by primes and themselves. I'd like to ensure primes are excluded from the output.

$n$ would be the input. A few examples to clarify;

$n=1$ returns 4, $n=2$ returns 6, $n=3$ returns 9, $n=4$ returns 10, $n=5$ returns 14, etc.

Help would be greatly appreciated!

EDIT: Turns out to be sequence A001358 in OEIS, thanks Raymond

edited Mar 19 '16 at 08:13

asked Mar 19 '16 at 05:46

Chip Wolf

Wouldn't $n=1$ return $4$? – John Douma Mar 19 '16 at 06:04
@JohnDouma Yep, oops. – Chip Wolf Mar 19 '16 at 06:05
1

So you are looking for numbers of the form $p_1p_2$ for $p_1,p_2$ prime. – Cheerful Parsnip Mar 19 '16 at 06:10
@GrumpyParsnip not a mathematician, could you explain the subscript after $p$? – Chip Wolf Mar 19 '16 at 06:18
If you are interesed by semiprimes (as suggested by Grumpy Parsnip) then see OEIS A251728 (your sequence) or perhaps A001358. – Raymond Manzoni Mar 19 '16 at 08:08
@RaymondManzoni exactly what I was after, thanks! – Chip Wolf Mar 19 '16 at 08:11
Glad it helped Chip and fine continuation, – Raymond Manzoni Mar 19 '16 at 08:13
@RaymondManzoni just grabbed that from the Wiki, thanks. Looks like the first one was the sequence I needed, TIL semiprimes are a thing – Chip Wolf Mar 19 '16 at 08:15

How to develop a function which returns the 'n'th number divisible only by primes (excluding primes)?

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