A081330 Numbers that can be written as sum of two 3-smooth numbers.
2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 48, 49, 50, 51, 52, 54, 55, 56, 57, 58, 59, 60, 62, 63, 64, 65, 66, 67, 68, 70, 72, 73, 74, 75, 76, 78, 80, 81, 82
Offset: 1
Keywords
Links
- Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
- Ivars Peterson, Medieval Harmony. [Broken link]
- Eric Weisstein's World of Mathematics, Smooth Number.
Programs
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Mathematica
max = 100; Table[2^i *3^j + 2^k* 3^l, {i, 0, Log[2, max] // Ceiling}, {j, 0, Log[3, max] // Ceiling}, {k, 0, Log[2, max] // Ceiling}, {l, 0, Log[3, max] // Ceiling}] // Flatten // Union // Select[#, # <= max &] & (* Jean-François Alcover, Sep 10 2017 *) -
PARI
list(lim)=my(v=List(), u=v, N); for(n=0, log(lim\1+.5)\log(3), N=3^n; while(N<=lim, listput(v, N); N<<=1)); v=Set(v); for(i=1,#v,for(j=i,#v, N=v[i]+v[j]; if(N>lim, break); listput(u,N))); Set(u) \\ Charles R Greathouse IV, Sep 07 2014
Formula
a(n) >> exp(n^(1/4)). - Charles R Greathouse IV, Aug 22 2011
Comments