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 A120316 Consecutive refactorable numbers a(n)-1, a(n) in which 5 is the smallest prime divisor of a(n). 0

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

%S 81450625,12594450625,29661450625,81924750625,195562950625,

%T 233313150625,308052750625,379733250625,764269350625,1490902050625,

%U 4606281750625,4995336750625,5551796250625,7502257950625,8450794350625,25857479250625

%N Consecutive refactorable numbers a(n)-1, a(n) in which 5 is the smallest prime divisor of a(n).

%F a(n) is the first integer of the form (5*k)^(5-1) such that both a(n) and a(n)-1 is refactorable and 5 is the smallest prime divisor of a(n).

%p with(numtheory); RFC5:=[]: p:=ithprime(3): P:=[seq(ithprime(i),i=1..2)]; 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 RFC5:=[op(RFC5),n]; print(ifactor(n)); fi fi; od od;

%Y Cf. A033950, A036898, A114617.

%K nonn

%O 1,1

%A _Walter Kehowski_, Jun 20 2006

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Last modified September 11 15:58 EDT 2024. Contains 375836 sequences. (Running on oeis4.)