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Template:Sequence of the Day for September 18

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Intended for: September 18, 2011

Timetable

  • First draft entered by Alonso del Arte on September 11, 2011
  • Draft reviewed by Daniel Forgues on September 18, 2012 ✓, September 18, 2018
  • Draft to be approved by August 18, 2011
Yesterday's SOTD * Tomorrow's SOTD

The line below marks the end of the <noinclude> ... </noinclude> section.



A020988: 
2
3
(4n  −  1) = 2 
4n  −  1
4  −  1
= 2 
n  − 1

k  = 0
4k =
n  − 1

k  = 0
2 2k +1, n   ≥   0
.
{ 0, 2, 10, 42, 170, 682, 2730, 10922, ... }
These are singly even numbers, i.e.
a (n)   ≡   2  (mod 4)
, thus with all 2s base 4 representation 2222...22224 (in the case of 0 we have zero 2s, which gives the impractical empty string more conveniently represented by showing a leading 0). These numbers are also the sum of distinct powers of 2 that are not powers of 4 (their binary representations have every odd indexed bit set to 1). In hexadecimal, these numbers are either all As or an initial 2 followed by all As. For example, 71582788216 in binary is 1010101010101010101010101010102 and in hexadecimal 2AAAAAAA16.