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‘Mine is a long and a sad tale!’ said the Mouse, turning to Alice, and sighing.‘It is a long tail, certainly’, said Alice, looking down with wonder at the Mouse’s tail; ‘but why do you call it sad?’
LEWIS CARROLL, Alice’s Adventures in Wonderland
We have already had (16·10,18·03) the functions
The corresponding series with all the signs taken positive are
The differential ...