合成數 (合成數) 或者合數 (合數) 是除了1佮自新以外,猶有其他育數存在的正整數。按照定義,每一个大於1个整數,若毋是數數,就是合成數。0佮1毋是數數,也毋是合成數。比論講,整數14是一个合成數,因為14會使被分解成2×7。
頭105个合成數是: 4,6,8,9,10,12,14,15,16,18,20,21,22,24,25,26,27,28,30,32,33,34,35,36,38,39,40,42,44,45,46,48,49,50,51,52,54,55,56,57,58,60,62,63,64,65,66,68,69,70,72,74,75,76,77,78,80,81,82,84,85,86,87,88,90,91,92,93,94,95,96,98,99,100,102,104,105,106,108,110,111,112,114,115,116,117,118,119,120,121,122,123,124,125,126,128,129,130,132,133,134,135,136,138,140。
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