A120317 - OEIS

%I #2 Mar 30 2012 18:36:04

%S 6080399213078595601,106451203123324908289,842122675409157900289,

%T 205035001401532317649921,690310240598397005456401,

%U 1125500133125681400538801,1241419580861102113344769

%N Consecutive refactorable numbers a(n)-1, a(n) in which 7 the smallest prime divisor of a(n).

%F a(n) is the first integer of the form (7*k)^(7-1) such that both a(n) and a(n)-1 is refactorable and 7 is the smallest prime divisor of a(n).

%p with(numtheory); RFC7:=[]: p:=ithprime(4): P:=[seq(ithprime(i),i=1..3)]; for w to 1 do for k from 3 to 12^4 by 2 do if andmap(z -> k mod z <> 0, P) then m:=p*k; n:=m^(p-1); t:=tau(n); n1:=n-1; t1:=tau(n1); if (n mod t = 0) and (n1 mod t1 = 0) then RFC7:=[op(RFC7),n]; print(ifactor(n)); fi fi; od od;

%Y Cf. A033950, A036898, A114617.

%K nonn

%O 1,1

%A _Walter Kehowski_, Jun 20 2006