OFFSET
1,1
COMMENTS
From David A. Corneth, Sep 28 2019: (Start)
If 10*m is in the sequence then so is 100*m.
The sum of squares of digits of a k-digit number is at most 81*k. Therefore any term with at most k digits is p-smooth where p is the largest prime < (81*k)^(1/2). (End)
LINKS
David A. Corneth, Table of n, a(n) for n = 1..14898 (terms < 10^20)
EXAMPLE
The prime factors of 4992 are 2,3,13, the sum of whose squares = 182 = sum of the squares of 4,9,9,2; so 4992 is a term of the sequence.
MATHEMATICA
f[n_] := Module[{a, l, t, r}, a = FactorInteger[n]; l = Length[a]; t = Table[a[[i]][[1]], {i, 1, l}]; r = Sum[(t[[i]])^2, {i, 1, l}]]; g[n_] := Module[{b, m, s}, b = IntegerDigits[n]; m = Length[b]; s = Sum[(b[[i]])^2, {i, 1, m}]]; Select[Range[2, 10^5], f[ # ] == g[ # ] &]
Select[Range[2, 4300000], Total[Transpose[FactorInteger[#]][[1]]^2]== Total[ IntegerDigits[#]^2]&] (* Harvey P. Dale, Sep 01 2011 *)
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Joseph L. Pe, Feb 18 2002
EXTENSIONS
a(16)-a(32) from Donovan Johnson, Sep 29 2009
STATUS
approved