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A235710
Composite numbers k such that sum of the proper divisors of k is a power of 10.
0
14, 124, 194, 1324, 1994, 13324, 133324, 1130324, 1333324, 13333324, 62496048, 133333324, 92782317392
OFFSET
1,1
COMMENTS
Composite numbers k such that reversal(sigma(k)-k) = 1.
If k is prime then sum of the proper divisors of k is 10^0.
If m is a natural number and p=10^m-3 is prime then 2*p is in the sequence.
If m is a natural number and p=(10^m-7)/3 is prime then 4*p is in the sequence.
a(14) > 3*10^12. - Giovanni Resta, Mar 21 2014
EXAMPLE
sigma(14)-14 = 1+2+7 = 10, sigma(124)-124 = 1+2+4+31+62 = 100.
MATHEMATICA
r[n_]:=FromDigits[Reverse[IntegerDigits[n]]]; Do[If[!PrimeQ[n]&& r[DivisorSigma[1, n]-n]==1, Print[n]], {n, 200000000}]
CROSSREFS
Sequence in context: A163942 A206628 A370966 * A239544 A167567 A188411
KEYWORD
nonn,base,more,changed
AUTHOR
Farideh Firoozbakht, Mar 17 2014
EXTENSIONS
a(13) from Giovanni Resta, Mar 21 2014
STATUS
approved