A036302 Composite numbers k such that the digits of the prime factors of k are either 1 or 2.
4, 8, 16, 22, 32, 44, 64, 88, 121, 128, 176, 242, 256, 352, 422, 484, 512, 704, 844, 968, 1024, 1331, 1408, 1688, 1936, 2048, 2321, 2662, 2816, 3376, 3872, 4096, 4222, 4442, 4642, 5324, 5632, 6752, 7744, 8192, 8444, 8884, 9284, 10648, 11264, 13504, 14641, 15488, 16384
Offset: 1
Examples
422 = 2 * 211 is in the sequence as the digits of its prime factors 2 and 211 are either 1 or 2. - _David A. Corneth_, Sep 26 2020
Links
- David A. Corneth, Table of n, a(n) for n = 1..10000 (first 1000 terms from Alois P. Heinz)
- Index entries sequences related to prime factors
Programs
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Magma
[k:k in [2..15000]| not IsPrime(k) and forall{a: a in PrimeDivisors(k)|Intseq(a) subset {1,2}}]; // Marius A. Burtea, Oct 08 2019 -
Mathematica
Select[Range[2,14650],!PrimeQ[#] && Complement[Flatten[IntegerDigits[First/@FactorInteger[#]]],{1,2}]=={} &] (* Jayanta Basu, May 25 2013 *)
Formula
Sum_{n>=1} 1/a(n) = Product_{p in A020450} p/(p-1) - Sum_{p in A020450} 1/p - 1 = 0.616325... - Amiram Eldar, Oct 14 2020
Comments