1 | initial version |

The question is an explicit one about how to compare two lists and make a third one out of them.

However, when programming some general considerents have priority. For instance, keep things simple.
In this example case, the operation of associating the list `L3`

is really not indicated, what shall we do with its elements? (And even if... why not directly `L3 = range(1,100)`

? And why redefine the one letter variable `i`

which traditionally stays for $\sqrt{-1}$. This was the first point. Then there is an internal function computing the sum of divisors. Why not use it? Shall we consider that we have to first compute all divisors, than add them is the quick way to do the job?

Why do we need `L2`

? Just to see we can associate this list as an example?!

An alternative way to get `L4`

without using list comprehension would be for example in sage:

```
L4 = []
for k in [1..100]:
s = sigma(k) - k
if s > k:
L4.append( s )
```

print L4

This gives:

```
[16, 21, 22, 36, 42, 55, 50, 54, 76, 66, 64, 108, 78, 74, 123, 90, 106, 140, 92, 144, 156, 117]
```

(And of course, now we do not know which sum comes from which $k$...)

Note: This post may not answer the question directly, but it is one in a series of posts, where we simply ignore there is already a function `sigma`

doing half of the job.

2 | No.2 Revision |

The question is an explicit one about how to compare two lists and make a third one out of them.

However, when programming some general considerents have priority. For instance, keep things simple.
In this example case, the operation of associating the list `L3`

is really not indicated, what shall we do with its elements? (And even if... why not directly `L3 = range(1,100)`

? And why redefine the one letter variable `i`

which traditionally stays for $\sqrt{-1}$. This was the first point. Then there is an internal function computing the sum of divisors. Why not use it? Shall we consider that we have to first compute all divisors, than add them is the quick way to do the job?

Why do we need `L2`

? (As a list, why do we not work elementwise.) Just to see we can associate this list as an example?!

An alternative way to get `L4`

without using list comprehension would be for example in sage:

```
L4 = []
for k in [1..100]:
s = sigma(k) - k
if s > k:
# print "%s = %s -> %s" % ( k, k.factor(), s )
L4.append( s )
print L4
```

print L4

This gives:

```
[16, 21, 22, 36, 42, 55, 50, 54, 76, 66, 64, 108, 78, 74, 123, 90, 106, 140, 92, 144, 156, 117]
```

(And of course, now we do not know which sum comes from which ~~$k$...)~~$k$... We could use in the above some intermediate prints to catch this information. Then why associate a list of $100$ elements, when finally their number is smaller?)

Note: This post may not answer the question directly, but it is one in a series of posts, where we simply ignore there is already a function `sigma`

doing half of the ~~job. ~~

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