On the doubly infinite products
Thomas S. Fiske · Bulletin of the American Mathematical Society · 1891
THE familiar singly infinite products for the sine and cosine, due to Euler,f sm -(-?)('-£) ('-é)--A 4a; 3 \ A 4x* \ A 4z" \ cos* = ^ -p-j (i -5p; (! -55?;-• • » or in another form, sm x + 00 |-COS X = JJ 1were first generalized by Abel.By a brilliant stroke of genius he obtained for the elementary doubly periodic functions the remarkable expressions % xllll ,_ moo + m'o/ sn a; =