OFFSET
1,1
COMMENTS
The terms are consecutive pairs, ordered so that (A) a(2i-1) < a(2i) for i > 0, and (B) a(2i+1) < a(2j+1) for 0 <= i < j. Problem 3 in section 7.2 of Burton's book asks the reader to prove a special case of this. - Jud McCranie, Dec 23 2018
REFERENCES
David Burton, Elementary Number Theory, 4th edition, McGraw-Hill, 1998, section 7.2, problem 3.
LINKS
Jud McCranie, Table of n, a(n) for n = 1..5000
Vladimir Letsko, Mathematical Marathon at VSPU (in Russian)
Vladimir Letsko, Mathematical Marathon at dxdy (in Russian)
EXAMPLE
phi(568) = phi(638) = 280; sigma(568) = sigma(638) = 1080; d(538) = d(638) = 8, so 568 and 638 are in the sequence. - Jud McCranie, Dec 23 2018
MATHEMATICA
Select[Values@ PositionIndex@ Array[Append[DivisorSigma[{0, 1}, #], EulerPhi@ #] &, 12500], Length@ # == 2 &] // Flatten (* Michael De Vlieger, Feb 17 2019 *)
PROG
(PARI) isok(n) = {s = sigma(n); ok = 0; if (s > n+1, v = vector(s-n+1, i, sigma(n+i)); for (i = 1, s-n+1, if (v[i] == s, npot = n+i; if ((numdiv(n) == numdiv(npot)) && (eulerphi(n) == eulerphi(npot)), return (npot); ); ); ); ); return (0); } \\ Michel Marcus, Oct 12 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladimir Letsko, Sep 28 2008, Sep 30 2008, Oct 17 2008
EXTENSIONS
a(42) from Michel Marcus, Oct 12 2013
STATUS
approved