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which runs about 25 times faster than my R solution! (And he further designed a 100 times faster version…)

Jamie also proposed an R code for solving the first version of that puzzle:

max = 1e10
squares = (1:floor(sqrt(max)))^2
# possible answers to a(a+6)
a = -1e6:1e6
# which squares have solutions
sols = intersect(a*(a + 6), squares)
# what are they?
f = function(x){
power = floor(floor(log10(x))/2)+1
a = -10^power:10^power
sols = c(x,a[a*(a+6) - x == 0])
names(sols) = c("square", "a1", "a2")
sols
}
sapply(sols,f)
## [,1]
## square 16
## a1 -8
## a2 2

which returns again 2 as the unique positive solution (equivalent to -8, if considering relative integers). A great lesson in efficient R programming, thanks Jamie!