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

81450625, 12594450625, 29661450625, 81924750625, 195562950625, 233313150625, 308052750625, 379733250625, 764269350625, 1490902050625, 4606281750625, 4995336750625, 5551796250625, 7502257950625, 8450794350625, 25857479250625

Offset

1,1

Links

Table of n, a(n) for n=1..16.

Formula

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).

Maple

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;

Crossrefs

Cf. A033950, A036898, A114617.

Sequence in context: A119075 A136965 A156943 * A118079 A186091 A295465

Adjacent sequences: A120313 A120314 A120315 * A120317 A120318 A120319

Keyword

nonn

Author

Walter Kehowski, Jun 20 2006

Status

approved