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直线数(英语:straight-line numbers)是大于99且其各位数构成等差级数的整数,例如,12345(公差为1),666(公差为0)或852(公差为-3)。[1][注 1]
尽管存在无限多个直线数,但其中只有96个不是纯位数,最大的是9876543210。[1]
首几个直线数是111,123,135,147,159,210,222,234,246,258,321,333,345,357,369,420,432,444,456,468,531,543,555,567,579,630,642,654,666,678,741,753,765,777,789,840,852,864,876,888,951,963,975,987,999,1111,1234,1357(OEIS数列A135643)。[2]
同时是直线数与质数的数称为直线质数。首几个直线质数是11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97,4567,76543,23456789,1111111111111111111,11111111111111111111111(OEIS数列A167847)。[3]
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