A284920 Numbers with digits 2 and 4 only.

2, 4, 22, 24, 42, 44, 222, 224, 242, 244, 422, 424, 442, 444, 2222, 2224, 2242, 2244, 2422, 2424, 2442, 2444, 4222, 4224, 4242, 4244, 4422, 4424, 4442, 4444, 22222, 22224, 22242, 22244, 22422, 22424, 22442, 22444, 24222, 24224, 24242, 24244, 24422, 24424

Offset: 1

Author

Jaroslav Krizek, Apr 05 2017

Keywords

• nonn
• base

Comments

All terms are even.

Crossrefs

Cf. Numbers with digits 2 and k only for k = 0 - 1 and 3 - 9: A169965 (k = 0), A007931 (k = 1), A032810 (k = 3), this sequence (k = 4), A072961 (k = 5), A284632 (k = 6), A284921 (k = 7), A284922 (k = 8), A284923 (k = 9).

Programs

Magma

[n: n in [1..100000] | Set(IntegerToSequence(n, 10)) subset {2, 4}]

Mathematica

Flatten@ Array[FromDigits /@ Tuples[{2, 4}, #] &, 5] (* Michael De Vlieger, Apr 06 2017 *)

Formula

a(n) = 2 * A007931(n).