A307108 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.

105, 231, 627, 897, 935, 1581, 1729, 2465, 2967, 3525, 4123, 4301, 4715, 5487, 7035, 7685, 7881, 9717, 10707, 11339, 14993, 16377, 17353, 17655, 20213, 20915, 23779, 24765, 25327, 26331, 26765, 29341, 29607, 32021, 33335, 34881, 40587, 40807, 42585, 42911, 48635

Offset: 1

Author

Alex Ratushnyak, Mar 25 2019

Keywords

• nonn

Examples

			105 = 3*7*5 is a term, 5 = (3+7) / 2.
231 = 3*11*7 is a term, 7 = (3+11) / 2.
3525 = 3*47*25 is a term, 25 = (3+47) / 2.
		

Crossrefs

Cf. A020639, A006530.

Cf. A262723 (a squarefree subsequence).

Programs

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