Russian term (edited): �������� � ��������� ������

We use symmetry considerations to argue for the special importance of the weakly damped modes oscillating with twice the frequency of the critical mode, and those oscillating primarily with the "difference frequency" |n-m|*omega and the "sum frequency" (n+m)*omega. - первая ссылка

It is seen that the yield of difference frequency generation is much larger than that of the sum frequency generation. - вторая ссылка

Честно говоря, не понимаю, о чем речь, но слова вроде бы переводятся так:)

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Note added at 2003-11-26 21:08:52 (GMT)
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http://www.boulder.nist.gov/timefreq/general/pdf/1175.pdf