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A120320 RF(5): refactorable numbers with smallest prime factor 5. 0
625, 1500625, 9150625, 17850625, 37515625, 52200625, 73530625, 81450625, 174900625, 442050625, 577200625, 1171350625, 1766100625, 1838265625, 2136750625, 3049800625, 4931550625, 7573350625, 8653650625, 12594450625 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

See A033950 for references. For any prime p, p^(p-1) is the smallest element of RF(p), the refactorable numbers whose smallest prime factor is p. Thus 5^(5-1)=625 is the first element. Other elements would also be 5^4*17^4 or 5^16*17^4.

LINKS

Table of n, a(n) for n=1..20.

FORMULA

a(n) = odd square, 5 is the smallest prime factor and refactorable.

MAPLE

with(numtheory); RF5:=[]: p:=5: for w to 1 do for j from 1 to 12^5 do k:=2*j+1; if k mod 3 <> 0 and k mod p = 0 then n:=k^2; t:=tau(n); if (n mod t = 0) then RF5:=[op(RF5), n]; print(ifactor(n)); fi fi; od od;

CROSSREFS

Cf. A033950, A036896, A036897.

Sequence in context: A187456 A223318 A222278 * A013837 A050448 A045171

Adjacent sequences:  A120317 A120318 A120319 * A120321 A120322 A120323

KEYWORD

nonn

AUTHOR

Walter Kehowski, Jun 20 2006

STATUS

approved

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Last modified October 22 00:52 EDT 2019. Contains 328315 sequences. (Running on oeis4.)