OFFSET
1,2
COMMENTS
The last of these iterates is the value in A173430.
REFERENCES
Graeme L. Cohen and Herman J. J. te Riele, Iterating the sum-of-divisors function, Experimental Mathematics, 5 (1996), pp. 93-100.
Oystein Ore, Number Theory and Its History, 1988, Dover Publications, ISBN 0486656209, pp. 88-96.
LINKS
Leonard Eugene Dickson, History of the Theory of Numbers, Volume I, Divisibility and Primality, Carnegie Institution of Washington, 1919, Chapters II and X
EXAMPLE
Calculating sum-of-divisors ( ... sum-of-divisors ( sum-of-divisors ( 7 ) ) ... ) the iterates are 7, 8, 15, 24, ... .
The initial, consecutive, pairwise, coprime iterates are 7, 8, 15, and there are 3 of these, so a(7) = 3.
Here sigma ( 7 ) = 8, sigma ( sigma ( 7 ) ) = sigma ( 8 ) = 15, etc.
PROG
(PARI) a(n)=my(t, s); if(n==1, 1, while(1, s++; t=sigma(n); if(gcd(t, n)==1, n=t, return(s)))) \\ Charles R Greathouse IV, Feb 06 2012
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Walter Nissen, Feb 18 2010
STATUS
approved