A167781 - OEIS

%I #5 Dec 26 2023 12:20:21

%S 1,4,45,14,58,25,68,17,183,41,27,85,274,101,875,38,184,53,478,196,848,

%T 807,23,173,615,125,271,303,189,351,405,191,1035,667,596,218,915,133,

%U 1525,397,376,92,1191,449,3466,552,47,346,404,137

%N a(n) = smallest number b such that b+n is the first number after b with phi(phi(b)) = phi(phi(b+n)).

%C Phi is Euler's totient function in this context.

%o (PARI) /* brute force search */ ppu(b)= { filename = "c:/math/ppu.txt"; if (!b,b=100000); v = listcreate(b+1); for(x=1,b,listput(v,eulerphi(eulerphi(x)))); z=b\5; sv = listcreate(z+1); for(y=1,z,listput(sv,0)); ct = 0; for(i=1,b, vi = 1; j= i + vi; noMatch = 1; /*print("Here is ",i," and ",v[i]);*/ while( (noMatch==1) && (vi < z) && (j < b), if (v[i]==v[j], if (sv[vi] == 0, /*print("*** Matching on ",vi," for ",i);*/ sv[vi] = i; ct+=1; ); noMatch = 0; , vi += 1; j += 1 ) ); if(nomatch==1,print(i," couldn't find a match before ",j)) ); for(i=1,z, if (sv[i]!=0, print(i,") ",sv[i]); write(filename,sv[i],",") , return(0)) ) }

%Y Cf. A167766, A167767, A167768

%K easy,nonn

%O 1,2

%A _Fred Schneider_, Nov 11 2009

%E Edited by _N. J. A. Sloane_, Nov 12 2009