# what is the value "60" doing in A000028 ??

Meeussen Wouter (bkarnd) wouter.meeussen at vandemoortele.com
Tue Sep 11 17:50:57 CEST 2001

```same for "84" = 2 * 42 , and others.

Also remark that 76 does not appear in either sequence A000028 or A000379
and since it's 4*19 it belongs to A000379.

There is however a quite different construction that generates A000028 and
A000379
as they stand (with the lost "76" added). That one is based on the MoebiusMu
-analog for infinitary divisors, defined by forcing mu() into the role of
MoebiusMu :
the sum over the i-divisors of the function mu(n/i_d)*s[i_d] should
equal
the sum over the i-divisors of s[i_d]. Solve for mu().

Q: was the Name " a(n) is smallest number not of form a(i)a(j), i<j<n."

***********  in plain 'Hungarian', this 'different construction' is
*******************

bitty[k_]:=Union[Flatten[Outer[Plus,Sequence@@
({0,#}&/@Union[(2^Range[0,Floor[Log[2,k]]])
Reverse[IntegerDigits[k,2]]])]]];

iDivisors[k_Integer]:= (Times @@ (First[it]^(#1 /. z -> List)) & ) /@
Flatten[Outer[z,Sequence @@ bitty/@Last[it=Transpose[FactorInteger[k]]],
1]] ;
iDivisors[1]:={1}

SumOveriDivisors[f_,n_Integer]:=Plus@@(Map[f[#1,n]&,  iDivisors[n]  ]);
inverseimoebius[seq_]:=SumOveriDivisors[ seq[[#1]]  &, # ]& /@
Range[Length[seq]];

seq= s/@ Range[0, 632] ;
itry=SumOveriDivisors[\[Mu][#2/#1] seq[[#1]]  &, # ]& /@ Range[Length[seq]]
;
inv=inverseimoebius[itry];

]

the positions of "1" are A000028 and those of "-1" are A000379 (or the other
way 'round).

Wouter Meeussen
tel  +32 (0)51 332 124
fax +32 (0)51 332 175
mail: wouter.meeussen at vandemoortele.com

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