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URL: https://oeis.org/A068421

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A068421
Numbers k such that sigma(k) = 6*(k - phi(k)).
0
5, 637, 2225, 243484576473600, 846376055078400
OFFSET
1,1
COMMENTS
No other term < 3500000. - Emeric Deutsch, Mar 08 2006
a(4) > 10^12. - Giovanni Resta, Nov 03 2012
Benoit Cloitre found that 6630421777056000 is another term of the sequence. - Jud McCranie, Mar 24 2026
MAPLE
with(numtheory): a:=proc(n) if sigma(n)=6*(n-phi(n)) then n else fi end: seq(a(n), n=1..5000); # Emeric Deutsch, Mar 08 2006
CROSSREFS
Sequence in context: A256070 A203339 A253690 * A142535 A203475 A203701
KEYWORD
more,nonn,hard
AUTHOR
Benoit Cloitre, Mar 02 2002
EXTENSIONS
a(4)-a(5) by Jud McCranie, Jan 21 2026
STATUS
approved