Mathematician telephoned Euler's number after delay (8)

'mathematician' is the definition.
(I've seen this before)

'telephoned euler's number after delay' is the wordplay.
'telephoned' becomes 'rang' ('ring' can be a synonym of 'telephone').
'euler's number' becomes 'e' (Euler's number - base of natural logarithms).
'after' means one lot of letters go next to another.
'delay' becomes 'lag' (lag is a kind of delay).
'rang'+'e'='range'
'range' after 'lag' is 'LAGRANGE'.