Need help with sequences
Tanya Khovanova
tanyakh at TanyaKhovanova.com
Wed Dec 27 21:28:01 CET 2006
Hello Seqfans,
I am not very good with Mathematica and my computer is slow. I would like to suggest several sequences for which I have only two terms. If you are interested, please, calculate more and submit.
Number 1:
4624, 595968 - Numbers n such that the sum of the 4 to the powers of its digits equals n.
Comment: If you use 2 instead of 4, such numbers do not exist. If you use 3, the only number is 12.
Example: 4^4 + 4^6 + 4^2 + 4^4 = 256 + 4096 + 16 + 256 = 4624.
Math: Select[Range[10000000], Plus @@ (4^IntegerDigits[#]) == # &]
Number 2:
5986, 8614 - Numbers that together with their prime factors contain every non-zero digit exactly once.
Example: 5986 = 2*41*73.
Math: Select[Range[2, 1000000],
Sort[Join[IntegerDigits[#],
Flatten[IntegerDigits[Transpose[FactorInteger[#]][[1]]]]]] == {1, 2, 3, 4, 5, 6, 7, 8, 9} &]
Number 3:
10, 2328 Numbers n such that prime(prime(n)) starts with n.
Example: prime(10) = 29, prime(29) = 109, which starts with 10.
Math: Select[Range[100000],
Prime[Prime[#]] - #*10^(Length[IntegerDigits[Prime[Prime[#]]]] -
Length[IntegerDigits[#]]) ==
Mod[Prime[Prime[#]],
10^(Length[IntegerDigits[Prime[Prime[#]]]] -
Length[IntegerDigits[#]])] &]
Compare: A067928 Numbers n such that the digits of the n-th prime
begin with n.; A074978 Prime(prime(n)) ends with n.
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