Consider a fundamental concept in maths that was created to 'solve' a problem that simply couldn't be solved by any other approach (or maybe for some other reason). Now let's assume that this concept made no sense to the contemporary mathematicians whatsoever that they simply brushed it aside as a purely mathematical tool with $0$ physical significance. like the square root of $-1 , $ $ i$ or the Euler number maybe?. Anything that makes no sense to a present-day mathematical outsider really.
A couple of decades later after its birth, baam, a scientist realises that the only way to solve a paradox in physics was to use this very mathematical concept that had to seem to be abstract at the time. Is there any good example of this in history?
This is with respect to the idea of maths being seen as a subject with no real-life application. I wanted to disprove by saying that many mathematical concepts do have a real-life application (if we look back at the history), but it's just that we haven't found any use so far.
I am slightly aware that e and complex numbers like $i$ makes it into the world of electrical engineering though I don't remember where exactly. I also vaguely recollect topology space and an array of mathematical concepts coming into the world by the theory of general relativity and quantum mechanics. What I am really looking for is
(1) a concept in maths that initially seems to have had no use
(2) 70 years later it is been used in a formula (please do add the formula too; that's really the reason I came here)