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A070813
Fermat primes minus 3.
9
0, 2, 14, 254, 65534
OFFSET
1,2
COMMENTS
Even numbers 2m such that phi(gpf(x)) - gpf(phi(x)) = 2m for some x, where gpf(m) is the largest prime divisor of m and phi(m) = totient(m).
Solutions to A070812(x) = 0 are in A007283, for A070812(x) = 2 are in A070004.
FORMULA
a(n) = A000215(n) - 3.
MATHEMATICA
pf[x_] := Part[Reverse[Flatten[FactorInteger[x]]], 2] Do[s=EulerPhi[pf[n]]-pf[EulerPhi[n]]; If[ !Odd[s]&&Greater[s, 2], Print[{n, s}], {n, 3, 10000000}] (* Only 2, 254 and 65534 appear in printout of s. The sequence is provided by Union[{s}, {0, 2}] *)
PROG
(PARI) for(n=0, 4, if(ispseudoprime(t=2^(2^n)+1), print1(t-3", "))) \\ Charles R Greathouse IV, Apr 26 2012
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Labos Elemer, May 09 2002
STATUS
approved