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 A135652 Divisors of 28 (the 2nd perfect number), written in base 2. 11
 1, 10, 100, 111, 1110, 11100 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The number of divisors of the second perfect number is equal to 2*A000043(2)=A061645(2)=6. LINKS FORMULA a(n)=A018254(n), written in base 2. Also, for n=1 .. 6: If n<=(A000043(2)=3) then a(n) is the concatenation of the digit "1" and n-1 digits "0" else a(n) is the concatenation of A000043(2)=3 digits "1" and (n-1-A000043(2)) digits "0". EXAMPLE The structure of divisors of 28 (see A018254) ---------------------------------------------------------------------- n ... Divisor . Formula ....... Divisor written in base 2 ............ ---------------------------------------------------------------------- 1)......... 1 = 2^0 ........... 1 2)......... 2 = 2^1 ........... 10 3)......... 4 = 2^2 ........... 100 .... (The 2nd superperfect number) 4)......... 7 = 2^3 - 2^0 ..... 111 .... (The 2nd Mersenne prime) 5)........ 14 = 2^4 - 2^1 ..... 1110 6)........ 28 = 2^5 - 2^2 ..... 11100... (The 2nd perfect number) PROG (PARI) apply(n->fromdigits(binary(n)), divisors(28)) \\ Charles R Greathouse IV, Jun 21 2017 CROSSREFS For more information see A018254 (Divisors of 28). Cf. A000043, A000079, A000396, A000668, A019279, A061645, A061652. Sequence in context: A273245 A276349 A167502 * A035504 A072366 A119082 Adjacent sequences:  A135649 A135650 A135651 * A135653 A135654 A135655 KEYWORD base,nonn,fini,full,easy,less AUTHOR Omar E. Pol, Feb 23 2008, Mar 03 2008 STATUS approved

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Last modified October 20 04:37 EDT 2018. Contains 316378 sequences. (Running on oeis4.)