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A071780
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Smallest k > n such that there are exactly n pairs (x,y) (1 <= x <= y <= k) solutions of the equation: phi(xy)=sigma(x)+sigma(y).
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0
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1, 7, 12, 15, 21, 26, 30, 33, 38, 60, 63, 65, 120, 189, 240, 296, 344, 442, 480, 567, 960, 1476, 1701, 1920, 3840, 4448, 5103, 7680, 15309, 15360, 30720, 45927, 57128, 61440, 66944, 122880, 137781, 245760, 413343, 491520, 538448, 983040, 1051136, 1240029
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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PROG
| (PARI) for(n=1, 12, s=2; while(sum(i=1, s, sum(j=1, i, if(eulerphi(i*j)-sigma(i)-sigma(j), 0, 1)))<n, s++); print1(s, ", "))
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CROSSREFS
| Sequence in context: A177988 A072834 A138615 * A190720 A063303 A091215
Adjacent sequences: A071777 A071778 A071779 * A071781 A071782 A071783
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KEYWORD
| nonn
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AUTHOR
| Benoit Cloitre (benoit7848c(AT)orange.fr), Jun 04 2002
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EXTENSIONS
| More terms from Ryan Propper (rpropper(AT)stanford.edu), Sep 28 2005
a(24)-a(43) from Donovan Johnson (donovan.johnson(AT)yahoo.com), May 30 2010
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