1,1

This sequence is not the same as A051200. Contrary to Gardner's comment, the first terms of that sequence are not accidental and do lead somewhere. The largest factor, if prime, is a sequence of 3's followed by a 1 (333333331 is not prime).

Find the last instance of the largest prime factor of odd N <= 10^n-1 associated with A136021.

a(1)=31 because it is the largest factor of odd N <= 10^2-1. The value of odd N where this factor first occurs is 3*31 = 93.

UBASIC: 10 H=2:N=1:goto 120 20 'distinct prime factors of composites <=10^n 30 S=0:N=N+2:Z=N\2:print N; 50 for F=1 to Z:Q=N/F: if Q<>int(Q) then 80 60 S=S+F: if F=prmdiv(F) and F>1 then print F; :C=C+1:G=G+F 70 if F=prmdiv(F) and F>1 then if H<=F then H=F else H=H 80 next F 100 if N=10^1-1 or N=10^2-1 or N=10^3-1 or N=10^4-1 or N=10^5-1 or N=10^6-1 or N=10^7-1 then print G; H:stop 110 print C, G, H:C=0 120 goto 30

(PARI) a(n)=precprime(n/3)

Cf. A051200, A136021.

Sequence in context: A011545 A011546 A152276 this_sequence A051200 A136596 A014178

Adjacent sequences: A136021 A136022 A136023 this_sequence A136025 A136026 A136027

easy,nonn

Enoch Haga (Enokh(AT)comcast.net), Dec 12 2007

Clarified and extended by Charles R Greathouse IV (charles.greathouse(AT)case.edu), Oct 11 2009