I heard this application note deals with sin wave generation especially converting triangle signal to sin with diode or with special transistors topologies. I want to study/understand theses principles.
That app note puts it around 1975, the internet didn't exist back then, so it was never online to be removed. They used to publish all those app notes in a book, there is a (to be unnamed) site that someone must have scanned one of Nationals last AN books (1994) into a pdf (violating copyright), which is how I figured it's an AN from ~1975.
FYI the AN-115 is NOT in that version of the Linear Application Handbook.
Here is an example of this method. By varying the voltage going to the diodes, some middle position can be found which causes the diodes to create a reasonable semblance of a sinewave.
One stage of diodes is not sufficient to properly trim the waveform so it is sinelike. Additional stages of diodes and resistors can be added.