OFFSET
1,2
COMMENTS
Least prime factor of A007908(n). For 1 < n <= 5000, a(n) < A007908(n), but this should fail infinitely often (assuming standard heuristics). - Charles R Greathouse IV, Apr 10 2014
From Robert Israel, Aug 28 2015: (Start)
a(n) = 2 iff n is even.
a(n) = 3 iff n == 3 or 5 (mod 6).
a(n) = 5 iff n == 25 (mod 30). (End)
LINKS
Robert G. Wilson v, Table of n, a(n) for n = 1..1000 (first 120 terms from Robert Israel)
EXAMPLE
a(5)= 3, 3 is the smallest prime divisor of 12345.
MAPLE
C:= 1: A[1]:= 1:
for n from 2 to 100 do
C:= C*10^(1+ilog10(n))+n;
F:= map(t -> t[1], ifactors(C, 'easy')[2]);
if hastype(F, integer) then A[n]:= min(select(type, F, integer))
else A[n]:= min(numtheory:-factorset(C))
fi
od:
seq(A[n], n=1..100); # Robert Israel, Aug 28 2015
MATHEMATICA
a = {}; b = {}; Do[w = RealDigits[n]; w = First[w]; Do[AppendTo[a, w[[k]]], {k, Length[w]}]; p = FromDigits[a]; AppendTo[b, First[First[FactorInteger[ p]]]], {n, 25}]; b (* Artur Jasinski, Apr 04 2008 *)
PROG
(PARI) lpf(n)=forprime(p=2, 1e3, if(n%p==0, return(p))); factor(n)[1, 1]
print1(N=1); for(n=2, 100, N=N*10^#Str(n)+n; print1(", "lpf(N))) \\ Charles R Greathouse IV, Apr 10 2014
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Amarnath Murthy, Sep 01 2002
EXTENSIONS
More terms from Sascha Kurz, Jan 03 2003
STATUS
approved