A307108 - OEIS

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

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OFFSET

1,1

LINKS

Table of n, a(n) for n=1..41.

EXAMPLE

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.

PROG

(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

CROSSREFS