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A073694
Numbers k such that the number of divisors of k equals the number of anti-divisors of k.
2
5, 32, 50, 162, 512, 1984, 2450, 3784, 5408, 7564, 9248, 15488, 19208, 22684, 26680, 30752, 53792, 79600, 85698, 102604, 113764, 131584, 189112, 199712, 279752, 336200, 435244, 514098, 546012, 581042, 658952, 712818, 727218, 752764, 767560
OFFSET
1,1
COMMENTS
See A066272 for definition of anti-divisor.
LINKS
Vincenzo Librandi and Donovan Johnson, Table of n, a(n) for n = 1..1000 (first 227 terms from Vincenzo Librandi)
EXAMPLE
32 is here since it has 6 divisors: {1, 2, 4, 8, 16, 32} and 6 anti-divisors: {3, 5, 7, 9, 13, 21}.
MATHEMATICA
atd[n_] := Count[Flatten[Quotient[#, Rest[Select[Divisors[#], OddQ]]] & /@ (2 n + Range[-1, 1])], Except[1]]; Select[Range[770000], DivisorSigma[0, #] == atd[#] &] (* Jayanta Basu, Jul 06 2013 *)
PROG
(PARI) {for(n=1, 770000, v1=[]; v2=[]; v3=[]; ds=divisors(2*n-1); for(k=2, matsize(ds)[2]-1, if(ds[k]%2>0, v1=concat(v1, ds[k]))); ds=divisors(2*n); for(k=2, matsize(ds)[2]-1, if(ds[k]%2>0, v2=concat(v2, ds[k]))); ds=divisors(2*n+1); for(k=2, matsize(ds)[2]-1, if(ds[k]%2>0, v3=concat(v3, ds[k]))); v=vecsort(concat(v1, concat(v2, v3))); if(matsize(v)[2]==numdiv(n), print1(n, ", ")))}
CROSSREFS
Sequence in context: A247549 A140154 A350997 * A322952 A101966 A341423
KEYWORD
nonn
AUTHOR
Jason Earls, Aug 30 2002
EXTENSIONS
Edited and extended by Klaus Brockhaus, Sep 01 2002
STATUS
approved