|
|
A110073
|
|
Numbers n such that sigma(n)=2n-phi(phi(n)).
|
|
2
|
|
|
1, 2, 4, 10, 76, 410, 890, 1370, 2330, 5690, 8090, 8570, 10490, 10970, 11930, 14330, 19130, 21530, 27770, 32090, 34490, 35930, 38330, 39290, 40730, 44570, 47930, 49370, 52730, 54170, 60890, 64730, 65690, 68570, 74330, 75290, 75770, 78170
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,2
|
|
COMMENTS
|
If p is an odd prime and 8p+1 is prime then n=10(8p+1) is in the sequence because 2n-phi(phi(n))=20(8p+1)-16(p-1)=144p+3618*(8p+2)=sigma(n).
Conjecture: Each term which is greater than 76 of this sequence is of the form 80p+10 where both p and 8p+1 are primes.
|
|
LINKS
|
|
|
EXAMPLE
|
76 is in the sequence because sigma(76)=2*76-phi(phi(76)).
|
|
MATHEMATICA
|
Do[If[DivisorSigma[1, m] == 2m - EulerPhi[EulerPhi[m]], Print[m]], {m, 100000}]
|
|
PROG
|
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
STATUS
|
approved
|
|
|
|