```
object Euler44 extends Application {
def pent (n :Int) = n*(3*n-1)/2
val pents = Set() ++ List.range(1, 3000).map(pent)
def find (a :Int, b :Int) :Int = {
val pa = pent(a)
val pb = pent(b)
val d = pb-pa
val s = pa+pb
if (pents.contains(d) && pents.contains(s)) d
else if (a >= b) find(1, b+1)
else find(a+1, b)
}
println(find(1, 2))
}
```

This one's not especially elegant or clever (but it is fast). We just scanning increasing pairs of integers for a pair that meets our criteria and stop when we find it. We optimize by generating a â€œsufficiently large" list of pentagonal numbers and putting them in a set so that we can quickly test a sum or difference for pentagonality.