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Numbers x that are equal to lpf(x)*gpf(x)*(lpf(x)+gpf(x))/2, where lpf(x) < gpf(x) are the least and the greatest prime factors of x: A020639 and A006530.
1

%I #21 Aug 27 2022 21:31:11

%S 105,231,627,897,935,1581,1729,2465,2967,3525,4123,4301,4715,5487,

%T 7035,7685,7881,9717,10707,11339,14993,16377,17353,17655,20213,20915,

%U 23779,24765,25327,26331,26765,29341,29607,32021,33335,34881,40587,40807,42585,42911,48635

%N Numbers x that are equal to lpf(x)*gpf(x)*(lpf(x)+gpf(x))/2, where lpf(x) < gpf(x) are the least and the greatest prime factors of x: A020639 and A006530.

%e 105 = 3*7*5 is a term, 5 = (3+7) / 2.

%e 231 = 3*11*7 is a term, 7 = (3+11) / 2.

%e 3525 = 3*47*25 is a term, 25 = (3+47) / 2.

%o (PARI) is(n) = my(f = factor(n)); omega(f) > 2 && (f[1, 1] * f[#f~, 1]) * (f[1, 1]+f[#f~, 1]) == n << 1 \\ _David A. Corneth_, Mar 25 2019

%Y Cf. A020639, A006530.

%Y Cf. A262723 (a squarefree subsequence).

%K nonn

%O 1,1

%A _Alex Ratushnyak_, Mar 25 2019