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A046433 Numbers such that 3 steps are needed to reach a prime under "Sum of digits raised to its digits' powers" procedure. 2
7, 26, 62, 126, 147, 148, 158, 162, 174, 184, 185, 206, 216, 248, 260, 261, 266, 284, 377, 399, 407, 408, 417, 418, 428, 470, 471, 480, 481, 482, 508, 518, 580, 581, 599, 602, 612, 620, 621, 626, 662, 704, 714, 737, 740, 741, 773, 804, 805, 814, 815, 824 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
LINKS
EXAMPLE
26 -> 2^2 + 6^6 = 46660 -> 4^4 + 6^6 + 6^6 + 6^6 + 0^0 = 140225 -> 1^1 + 4^4 + 0^0 + 2^2 + 2^2 + 5^5 = 3391, which is prime, so 26 is a term.
MATHEMATICA
sop[n_] := Total[#^# & /@ (IntegerDigits[n] /. {0 -> 1})]; Select[ Range[825], Boole /@ PrimeQ[Rest[NestList[sop, #, 3]]] == {0, 0, 1} &] (* Jayanta Basu, Jun 25 2013 *)
PROG
(Magma) f:=func<n| &+[a[i]^a[i]:i in [1..#a]] where a is Intseq(n) >; [k:k in [1..900] |not IsPrime(f(k)) and not IsPrime(f(f(k))) and IsPrime(f(f(f(k))))]; // Marius A. Burtea, Oct 27 2019
CROSSREFS
Cf. A046431.
Sequence in context: A231888 A211645 A171340 * A128972 A135300 A024001
KEYWORD
nonn,base
AUTHOR
Patrick De Geest, Jul 15 1998
STATUS
approved

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Last modified May 22 15:18 EDT 2024. Contains 372758 sequences. (Running on oeis4.)