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A126160
Number of betrothed pairs (m,n) with m <=10^k (and k=1,2,3,...), where a betrothed pair satisfies sigma(m)=sigma(n)=m+n+1 and m<n.
1
0, 1, 2, 8, 9, 17, 46, 79, 180, 404, 882, 1946, 4122
OFFSET
1,3
COMMENTS
Also called quasi-amicable pairs, or reduced amicable pairs.
LINKS
J. O. M. Pedersen, Tables of Aliquot Cycles [Broken link]
J. O. M. Pedersen, Tables of Aliquot Cycles [Via Internet Archive Wayback-Machine]
J. O. M. Pedersen, Tables of Aliquot Cycles [Cached copy, pdf file only]
P. Pollack, Quasi-Amicable Numbers are Rare, J. Int. Seq. 14 (2011) # 11.5.2.
EXAMPLE
a(7)=46 because there are 46 betrothed pairs (m,n) with m<=10^7
MATHEMATICA
s[n_]:=DivisorSigma[1, n]-n; BetrothedNumberQ[n_]:=If[s[s[n]-1]==n+1 && n>1, True, False]; BetrothedPairList[k_]:=(anlist=Select[Range[k], BetrothedNumberQ[ # ] &]; prlist=Table[Sort[{anlist[[n]], s[anlist[[n]]]-1}], {n, 1, Length[anlist]}]; Union[prlist, prlist]); data=BetrothedPairList[10^6]; Table[Length[Select[data, First[ # ]<10^k &]], {k, 1, 6}]
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Ant King, Dec 19 2006
EXTENSIONS
a(13) from Giovanni Resta, Jul 24 2019
STATUS
approved