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## Main Question or Discussion Point

If for some functional ##I##, ##\delta I=0## where ##\delta## is symbol for variation functional has extremum. For ##\delta^2 I>0## it is minimum, and for ##\delta^2 I<0## it is maximum. What if

##\delta I=\delta^2 I=0##. Then I must go with finding further variations. And if ##\delta^3I>0## is then that minimum? Or what?

##\delta I=\delta^2 I=0##. Then I must go with finding further variations. And if ##\delta^3I>0## is then that minimum? Or what?