A120317 Consecutive refactorable numbers a(n)-1, a(n) in which 7 the smallest prime divisor of a(n).

6080399213078595601, 106451203123324908289, 842122675409157900289, 205035001401532317649921, 690310240598397005456401, 1125500133125681400538801, 1241419580861102113344769

Offset

1,1

Links

Table of n, a(n) for n=1..7.

Formula

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).

Maple

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;

Crossrefs

Cf. A033950, A036898, A114617.

Sequence in context: A003812 A003805 A115530 * A219283 A306797 A213529

Adjacent sequences: A120314 A120315 A120316 * A120318 A120319 A120320

Keyword

nonn

Author

Walter Kehowski, Jun 20 2006

Status

approved