A099433 Numbers n such that A066421(k) < A066421(n) for all k < n.
1, 2, 4, 33, 111, 603, 804, 1153, 12759, 36090, 114913
Offset: 1
Extensions
a(10)-a(11) from Donovan Johnson, Nov 22 2008
This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
For n=12, sigma^{4}(12)-1 = 360-1 = 359 is prime, and there is no positive k<4 such that sigma^{k}(12)-1 is prime, so a(12)=4.
sigma_iterate := proc (n, k)
local sigma_result, i:
sigma_result := n:
for i to k do
sigma_result := sigma(sigma_result)
end do:
return sigma_result
end proc:
find_min_k := proc (n)
local k, sigma_k_n, prime_candidate:
k := 0:
do
k := k+1:
sigma_k_n := sigma_iterate(n, k):
prime_candidate := sigma_k_n - 1:
if isprime(prime_candidate) then
return k
end if
end do
end proc:
map(find_min_k, [$ 2 .. 100]);
A374163[n_] := If[n==1, 1, Length[NestWhileList[DivisorSigma[1, #]&, n, !PrimeQ[# - 1]&, {2, 1}]] - 1]; Array[A374163, 100] (* Paolo Xausa, Jul 24 2024 *)
a(n) = my(k=1, s=sigma(n)); while(!isprime(s-1), k++; s = sigma(s)); k; \\ Michel Marcus, Jun 29 2024
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