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Math Help - partial sum

  1. #1
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    partial sum

    given that the seq {an} is non increasing and an tends to 0, i need to show that sum from n=1 to infinity of (-1)^n an conversed. this is the alternating series theorem..


    i was reading its proof and wondering,

    if we consider only the even terms of the seq,

    then S2n= -a1 + (a2 -a3) +(a4... = by paring, all pairs of terms are positive except -a1

    then does it mean that S2n is increasing and hence bounded below by -a1?

    then we cant say that the partial sum converges right?

    may i know what is wrong with my understanding?


    (my notes state that by the monotone convergence thm, S2n converges to a limit as n tend to infinity)
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  2. #2
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    Quote Originally Posted by alexandrabel90 View Post
    given that the seq {an} is non increasing and an tends to 0, i need to show that sum from n=1 to infinity of (-1)^n an conversed. this is the alternating series theorem..


    i was reading its proof and wondering,

    if we consider only the even terms of the seq,

    then S2n= -a1 + (a2 -a3) +(a4... = by paring, all pairs of terms are positive except -a1

    then does it mean that S2n is increasing and hence bounded below by -a1?

    then we cant say that the partial sum converges right?

    may i know what is wrong with my understanding?


    (my notes state that by the monotone convergence thm, S2n converges to a limit as n tend to infinity)


    Google "Leibnitz series"...

    Tonio
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