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Difference between primes p and the largest prime divisor of p-1.
1

%I #19 Sep 08 2016 03:31:01

%S 3,4,6,10,15,16,12,22,26,34,36,36,24,40,30,56,56,64,70,66,42,78,94,96,

%T 86,54,106,106,120,118,120,116,112,146,144,160,84,130,90,176,172,190,

%U 190,188,204,186,114,210,204,222,236,246,255,132,202,266,254,274,236

%N Difference between primes p and the largest prime divisor of p-1.

%C This sequence contains only one prime number, the 3.

%e For prime 107 we have 107 - 53 = 54.

%p A085378 := proc(n)

%p ithprime(n)-A023503(n) ;

%p end proc:

%p seq( A085378(n),n=3..80) ; # _R. J. Mathar_, Sep 07 2016

%t #-FactorInteger[#-1][[-1,1]]&/@Prime[Range[3,70]] (* _Harvey P. Dale_, Jan 04 2012 *)

%o (PARI) cminuspm1(n) = \ prime - maxprime of prime-1 { forprime(x=5,n, forstep(p=x,2,-1, if(isprime(p) & (x-1)%p==0,print1(x-p,","); break); ) ) }

%K easy,nonn

%O 3,1

%A _Cino Hilliard_, Aug 12 2003