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I look for an argument to this statement:

$$ \int_a^x dx_1 \int_a^{x_1} K(x_1,t) dt= \int_a^xdt \int_t^x K(x_1,t) dx_1 $$ It is certainly an integration by change of variables that I can not clarify

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    Do you see the domain..?2012-03-19
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    $K\ge0$? $\,\,\,\,$2012-03-19

1 Answers 1