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A257754
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Quasi-Carmichael numbers to exactly four bases.
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10
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60491, 61937, 65311, 76151, 116843, 127723, 159197, 164009, 168821, 194417, 272483, 284987, 329467, 364087, 369857, 370817, 385241, 389327, 395497, 407837, 423701, 431393, 465043, 509461, 613927, 837209, 853607, 881717, 999919, 1041541, 1117213, 1279903, 1294819
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OFFSET
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1,1
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LINKS
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EXAMPLE
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a(1) = 60491 because this is the first squarefree composite number n such that exactly four integers b except 0 exist such that for every prime factor p of n, p+b divides n+b (-239, -236, -231, -191): 60491=241*251 and 2, 12 both divide 60252 and 5, 15 both divide 60255 and 10, 20 both divide 60260 and 50, 60 both divide 60300.
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PROG
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(PARI) for(n=2, 1000000, if(!isprime(n), if(issquarefree(n), f=factor(n); k=0; for(b=-(f[1, 1]-1), n, c=0; for(i=1, #f[, 1], if((n+b)%(f[i, 1]+b)>0, c++)); if(c==0, if(!b==0, k++))); if(k==4, print1(n, ", ")))))
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CROSSREFS
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Cf. A257750 (every number of bases).
Cf. A257751, A257752, A257753, A257755, A257756, A257757, A258842 (1, 2, 3, 5, 6, 7 and 8 bases).
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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