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A120321 RF(7): refactorable numbers with 7 as smallest prime factor. 0
117649, 208422380089, 567869252041, 2839760855281, 5534900853769, 17416274304961, 69980368892329, 104413920565969, 301855146292441, 558845013849409, 743702041351801, 1268163904241521, 2607614922465721 (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 7^(7-1)=117649 is the first element. Other elements would also be 7^6*17^6 or 7^16*17^6. Here are the prime factorizations for the first 49 elements of RF7: (7^6), (7^6)*(11^6), (7^6)*(13^6), (7^6)*(17^6), (7^6)*(19^6), (7^6)*(23^6), (7^6)*(29^6), (7^6)*(31^6), (7^6)*(37^6), (7^6)*(41^6), (7^6)*(43^6), (7^6)*(47^6), (7^6)*(53^6), (7^6)*(59^6), (7^6)*(61^6), (7^6)*(67^6), (7^6)*(71^6), (7^6)*(73^6), (7^6)*(79^6), (7^6)*(83^6), (7^6)*(89^6), (7^12)*(13^6), (7^6)*(97^6), (7^6)*(101^6), (7^6)*(103^6), (7^6)*(107^6), (7^6)*(109^6), (7^6)*(113^6), (7^6)*(127^6), (7^6)*(131^6), (7^6)*(137^6), (7^6)*(139^6), (7^6)*(11^6)*(13^6), (7^6)*(149^6), (7^6)*(151^6), (7^6)*(157^6), (7^6)*(163^6), (7^6)*(167^6), (7^6)*(13^12), (7^6)*(173^6), (7^6)*(179^6), (7^6)*(181^6), (7^6)*(11^6)*(17^6), (7^6)*(191^6), (7^6)*(193^6), (7^6)*(197^6), (7^6)*(199^6), (7^6)*(11^6)*(19^6), (7^6)*(211^6).

LINKS

Table of n, a(n) for n=1..13.

FORMULA

a(n) = odd square, 7 is the smallest prime factor and refactorable.

EXAMPLE

a(1)=7^(7-1)=117649.

MAPLE

with(numtheory); p:=7: RF7:=[p^(p-1)]: P:=[seq(ithprime(i), i=2..pi(p)-1)]; for w to 1 do for j from 1 to 12^3 do k:=2*j+1; if andmap(z -> k mod z <> 0, P) then for s from 2 to p-1 by 2 do #accelerate creation n:=7^6*k^s; t:=tau(n); if not n in RF7 and (n mod t = 0) then RF7:=[op(RF7), n]; print(ifactor(n)); fi; od; fi; od od; RF7:=sort(RF7);

CROSSREFS

Cf. A033950, A036896, A036897.

Sequence in context: A017358 A017478 A017610 * A202599 A251882 A237208

Adjacent sequences:  A120318 A120319 A120320 * A120322 A120323 A120324

KEYWORD

nonn

AUTHOR

Walter Kehowski, Jun 21 2006

STATUS

approved

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Last modified October 22 22:34 EDT 2019. Contains 328335 sequences. (Running on oeis4.)