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A167781 a(n) = smallest number b such that b+n is the first number after b with phi(phi((b)) = phi(phi(b+n)). 0
1, 4, 45, 14, 58, 25, 68, 17, 183, 41, 27, 85, 274, 101, 875, 38, 184, 53, 478, 196, 848, 807, 23, 173, 615, 125, 271, 303, 189, 351, 405, 191, 1035, 667, 596, 218, 915, 133, 1525, 397, 376, 92, 1191, 449, 3466, 552, 47, 346, 404, 137 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Phi is Euler's totient function in this context.

LINKS

Table of n, a(n) for n=1..50.

PROG

(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)) ) }

CROSSREFS

Cf. A167766, A167767, A167768

Sequence in context: A243221 A238816 A024254 * A173327 A075029 A070225

Adjacent sequences:  A167778 A167779 A167780 * A167782 A167783 A167784

KEYWORD

easy,nonn

AUTHOR

Fred Schneider, Nov 11 2009

EXTENSIONS

Edited by N. J. A. Sloane, Nov 12 2009

STATUS

approved

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Last modified November 19 16:10 EST 2017. Contains 294936 sequences.