OFFSET
1,1
LINKS
Ivan Neretin, Table of n, a(n) for n = 1..10000
FORMULA
Conjecture: Sum_{k=1..n} a(k) = O(n^{2+epsilon}) for any epsilon > 0.
Between n = 90000 and 100000, Sum_{k=1..n} a(k)/n^2 slowly but not monotonically increases from 1.0007 to 1.0023. At n = 10^6, it's about 1.0147. - David A. Corneth, Oct 23 2017
MATHEMATICA
a = ConstantArray[1, 68]; k = 1; While[Length[vac = Flatten[Position[a, 1]]] > 0, k++; a[[Intersection[Divisors[DivisorSigma[1, k] - k], vac]]] *= k]; a (* Ivan Neretin, May 15 2015 *)
lk[n_]:=Module[{k=2}, While[!Divisible[DivisorSigma[1, k]-k, n], k++]; k]; Array[lk, 70] (* Harvey P. Dale, Oct 23 2017 *)
PROG
(PARI) a(n)=if(n<0, 0, s=2; while((sigma(s)-s)%n>0, s++); s)
(PARI) first(n)=my(res = vector(n), todo = n - 1, k = 2); res[1] = 2; while(todo > 0, d = divisors(sigma(k) - k); for(i=2, #d, if(d[i] <= n && res[d[i]] == 0, res[d[i]] = k; todo--)); k++); res \\ David A. Corneth, Oct 23 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Benoit Cloitre, Sep 17 2002
STATUS
approved