Not sure if this quite fits the criteria, but . . .

The Ramanujan-Nagell equation was introduced in 1913 by Ramanujan in the J. Indian Math. Soc. (Vol.5):

Question 464: $2^n - 7$ is a perfect square for the values $3$, $4$, $5$, $7$, $15$ of $n$. Find other values.

The question was asked independently in 1943 by W. Ljunggren in Norsk Mat. Tidsskr. (Vol.25), and answered by T.Nagell five years later in the same journal ("Løsning till oppgave nr 2"). Since that journal is in Norwegian, most of the community did not know of the result until 1961 when Nagell republished it in Ark. Mat. (Vol.30, "The Diophantine equation $x^2 + 7 = 2^n$").

The result is that there are no solutions other than the five that Ramanujan listed.