"Like Young", a short story by Theodore Sturgeon, first published in The Magazine of Fantasy and Science Fiction, March 1960, available at the Internet Archive.
We were, all of us, devoted to a single idea, and that was that Humanity should not perish. Humanity in the sense of aspiration, generosity—if you like, nobility; that was what we were dedicated to preserve. It was too late for us to use it. We’d only just realized what it was, when the new encephalitis appeared. Perhaps we realized it because the encephalitis appeared. However we came by it, we had it, and we had to pass it on, or it was all too ludicrous a tragedy.
We decided to give it to the otters.
[. . . .]
But we were determined that once it was burning it should never flicker or dim. There would be no dark ages for the otters. We would reduce basic knowledge to its essences, put these in the most understandable form, and leave them like milestones (a statement and a promise, each) along the way.
For the milestones we chose the new alloy 2-chrome-vanadium-prime, which came to be called bicrovalloy. (Ah, what cities that might have built!) Properly fabricated, it could be formed into rods, bars, sheets; once irradiated, it
would not, almost could not, change its form or state.
[. . . .]
The otter, when at last I had crept round the dais and up behind the curtains and could see him, crouched motionless before and between the two bicrovalloy plates, the one just recovered, bearing Einstein's and Heisenberg's revelations, and the other which had just been fabricated to replace it.
I thought (a very whisper of a thought, lest I think too loudly
and ripple this tableau): Are you praying, little one?
[. . . .]
In the brilliant moonlight I gazed down at the shrine of humanity, ll its dignity and its worth, and at all the meanings of this mighty gesture of faith in the life that had been and the life that was to be, when my eyes took in . . . took in what, some unmeasured time later—it might have been an hour—my mind was able to take in . . .
. . . just to the right of Einstein’s brief immortal perfect statement of mass-energy conversion, the comment,
written on, written into the bicrovalloy plate.
And there were two corrections in the Heisenberg statement, strikeouts and carelessly scribbled figures which seemed to have been scribed deep in the impervious metal by a single small foreclaw . . .
But it was what had been done to the new De Wald plate that dealt me that blinding blow, from which I recovered (was it an hour later?) so slowly. For under that climactic, breathtaking achievement of intuitive mathematics, that
most transcendental of all human statements, the De Wald Synthesis, the otter had scrawled: