A145098 Duplicate of A136031.
89, 178481, 2089, 616318177, 164511353, 2099863, 13264529, 20394401, 3203431780337, 761838257287, 212885833, 9361973132609
Offset: 1
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.
2^11 - 1 = 2047 = 23*89.
A065341 := proc(n) local i; i := 2^(ithprime(n))-1: if (not isprime(i)) then RETURN (i) fi: end: seq(A065341(n), n=1..21); # Jani Melik, Feb 09 2011
Select[Table[2^Prime[n]-1,{n,30}],!PrimeQ[#]&] (* Harvey P. Dale, May 06 2018 *)
a = {}; Do[m = 2^Prime[n] - 1; If[PrimeQ[m], null, AppendTo[a, Divisors[m][[ -2]]]], {n, 1, 40}]; a
N:= 24: # to get a(1) to a(N)
unknown:= N:
for k from 2 while unknown > 0 do
q:= numtheory:-bigomega(2^k-1);
if q <= N and not assigned(A[q]) then
A[q]:= k;
unknown:= unknown - 1;
fi
od:
seq(A[i],i=1..N); # Robert Israel, Oct 24 2014
Module[{nn=250,tbl},tbl=Table[{k,PrimeOmega[2^k-1]},{k,nn}];Table[SelectFirst[tbl,#[[2]]==n&],{n,24}]][[;;,1]] (* The program generates the first 24 terms of the sequence. *) (* Harvey P. Dale, May 25 2025 *)
a(n) = {k = 1; while(bigomega(2^k-1) != n, k++); k;} \\ Michel Marcus, Nov 04 2013
a = {}; Do[m = 2^Prime[n] - 1; k = Divisors[m][[ -2]]; If[PrimeQ[m], null, If[PrimeQ[k], AppendTo[a, 1], AppendTo[a, 0]]], {n, 1, 50}]; a
With[{pn1=PrimeNu[2^Range[800]-1]},Table[Position[pn1,n,1,1],{n,0,40}]]//Flatten (* Harvey P. Dale, Jan 10 2025 *)
a(n) = my(k=1); while (omega(2^k-1) != n, k++); k; \\ Michel Marcus, Jan 09 2023
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