OFFSET
1,1
COMMENTS
Inspired by A047983.
Are there prime terms greater than 31?
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
FORMULA
A047983(a(n)) = n. - Rémy Sigrist, Dec 06 2020
EXAMPLE
The smallest number having two smaller numbers (2 and 3) with the same number of divisors is 5, so a(2) is 5.
MAPLE
N:= 500: # for terms before the first term > N
T:= map(numtheory:-tau, [$1..N]):
M:= max(T):
V:= Vector(M):
for n from 1 to N do
v:= T[n];
V[v]:= V[v]+1;
if not assigned(R[V[v]]) then R[V[v]]:= n fi
od:
for nn from 1 while assigned(R[nn]) do od:
seq(R[i], i=2..nn-1); # Robert Israel, Oct 30 2020
MATHEMATICA
f[n_]:=With[{tau=DivisorSigma[0, n]}, Length[Select[Range[n-1], DivisorSigma[0, #]==tau&]]]; t=Table[f[n], {n, 1, 300}]; a[n_]:=FirstPosition[t, n]; Rest[a/@Range[0, 65]]//Flatten (* f(n) by Jean-François Alcover at A047983 *)
PROG
(PARI) f(n) = {my(d=numdiv(n)); sum(k=1, n-1, (numdiv(k)==d))} \\ A047983
a(n) = my(k=1); while (f(k)!= n, k++); k; \\ Michel Marcus, Oct 30 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Ivan N. Ianakiev, Oct 30 2020
STATUS
approved