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A126646
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a(n) is the number of integers k less than 10^n such that the decimal representation of k lacks the digits 1,2,3,4,5,6 and 7 and at least one of digits 8,9.
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3
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3, 7, 15, 31, 63, 127, 255, 511, 1023, 2047, 4095, 8191, 16383, 32767, 65535, 131071, 262143, 524287, 1048575, 2097151, 4194303, 8388607, 16777215, 33554431, 67108863, 134217727, 268435455, 536870911, 1073741823, 2147483647
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| [a(n-1)]^2+a(n) is a(2n)+1, a square. Ex:3^2+7=16; 7^2+15=64; 15^2+31=256; etc. [From Vincenzo Librandi and Ralf Stephan, Nov 23 2010]
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LINKS
| Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets
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FORMULA
| a(n)=2*2^n-1
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EXAMPLE
| a(8)=511
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MAPLE
| f:=n->2*2^n-1;
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CROSSREFS
| Cf. A125630, A125948, A125947, A125946, A125945, A125940, A125909, A125908, A125880, A125897, A125904, A125858.
Sequence in context: A097002 A060152 A168604 * A000225 A123121 A117060
Adjacent sequences: A126643 A126644 A126645 * A126647 A126648 A126649
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KEYWORD
| nonn
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AUTHOR
| Aleksandar M. Janjic and Milan R. Janjic (agnus(AT)blic.net), Feb 08 2007, Feb 13 2007
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