∞∑n=3 (1/n)*(1/lnn)*(1/lnlnn)的敛散性

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∞∑n=3(1/n)*(1/lnn)*(1/lnlnn)的敛散性∞∑n=3(1/n)*(1/lnn)*(1/lnlnn)的敛散性∞∑n=3(1/n)*(1/lnn)*(1/lnlnn)的敛散性设f(x

∞∑n=3 (1/n)*(1/lnn)*(1/lnlnn)的敛散性
∞∑n=3 (1/n)*(1/lnn)*(1/lnlnn)的敛散性

∞∑n=3 (1/n)*(1/lnn)*(1/lnlnn)的敛散性
设f(x) = 1/(x·ln(x)·ln(ln(x))),易见f(x)在(3,+∞)上单调递减.
根据Cauchy积分判别法,级数∑f(n)与广义积分∫{3,+∞}f(x)dx敛散性相同.
而∫ f(x)dx = ∫ 1/(x·ln(x)·ln(ln(x))) dx
= ∫ 1/(ln(x)·ln(ln(x))) d(ln(x))
= ∫ 1/ln(ln(x)) d(ln(ln(x)))
= ln(ln(ln(x)))+C,
当A → +∞时,∫{3,A} f(x)dx = ln(ln(ln(A)))-ln(ln(ln(3))) → +∞,
广义积分∫{3,+∞}f(x)dx发散,故级数∑f(n) = ∑1/(n·ln(n)·ln(ln(n)))也发散.