OneLiners.rb

def mean(set)
 set.length < 2 ? set[0] : (set[0] + mean set[1..set.length]) / set.length.to_f
end

def sigma(set)
 set.length < 2 ? set[0] : set[0] + sigma set[1..set.length]
end

def is_prime?(val, c=(val-1))
	c < 2 or c >= val ? false : (val/(c.to_f)) - (val/c) == 0 ? false : is_prime?(val, c - 1)
end