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semiprimes
A number is called semiprime if it is equal to the product of two primes. Semiprimes are sometimes also called 2-almost primes or biprimes.

There are infinite semiprimes and the sum of their reciprocals does not converge.

Brilliant numbers are semiprimes  $p\cdot q$  for which  $p$  and  $q$  have the same number of digits.

Chen Jingrun has proved that every sufficiently large even number can be written as the sum of either two primes, or a prime and a semiprime.

The first semiprimes are 4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 38, 39, 46, 49, 51, 55, 57, 58, 62, 65 more terms

Semiprimes can also be... (you may click on names or numbers and on + to get more values)

aban 10 14 15 + 99000998 alt.fact. 326981 36614981 alternating 10 14 21 + 98989834 amenable 21 25 33 + 99999997 apocalyptic 218 226 247 + 29999 arithmetic 14 15 21 + 9999998 astonishing 15 automorphic 25 Bell 15 203 binomial 10 15 21 + 99214741 brilliant 10 14 15 + 99460729 c.decagonal 361 451 551 + 99882151 c.heptagonal 22 106 253 + 99972881 c.nonagonal 10 55 91 + 99214741 c.octagonal 25 49 121 + 99460729 c.pentagonal 51 106 141 + 99619141 c.square 25 85 145 + 99983941 c.triangular 10 46 85 + 99964099 cake 15 26 93 + 89231899 Canada 581 8549 16999 Carol 959 261119 67092479 Catalan 14 congruent 14 15 21 + 9999998 constructible 10 15 34 + 16843009 Cullen 25 65 161 + 44040193 Cunningham 10 15 26 + 99960005 Curzon 14 21 26 + 99999794 cyclic 15 33 35 + 9999997 D-number 15 21 33 + 7043133 d-powerful 209 226 262 + 9999827 de Polignac 905 959 1199 + 99999937 decagonal 10 85 451 + 99945007 deceptive 91 259 451 + 99790381 deficient 10 14 15 + 9999998 dig.balanced 10 15 21 + 67092481 double fact. 15 Duffinian 21 25 35 + 9999997 eban 34 46 62 + 66064034 economical 10 14 15 + 19999982 emirpimes 15 26 39 + 99999997 equidigital 10 14 15 + 19999982 eRAP 26123 182939 1572827 + 97116947 esthetic 10 21 34 + 89878787 Eulerian 26 57 247 + 41932745 evil 10 15 33 + 99999997 fibodiv 14 122 183 + 97356793 Fibonacci 21 34 55 + 5702887 Friedman 25 121 289 + 999163 frugal 1369 1681 1849 + 99460729 gapful 121 143 187 + 99990607 Gilda 49 314 977909 + 4848955 happy 10 49 82 + 9999967 Harshad 10 21 111 + 99993563 heptagonal 34 55 235 + 99751747 hex 91 169 217 + 99965269 hexagonal 15 91 703 + 97071211 hoax 22 58 85 + 99999706 Hogben 21 57 91 + 99910021 house 155 933 1285 + 99402731 hungry 74 161449 2401519 hyperperfect 21 301 697 + 97585249 iban 10 14 21 + 777773 iccanobiF 39 514 61135 792517 idoneal 10 15 21 + 253 inconsummate 62 65 95 + 999995 insolite 111 interprime 15 21 26 + 99999843 Jacobsthal 21 85 341 + 22369621 junction 111 115 202 + 99999857 Kaprekar 55 703 181819 + 5072059 katadrome 10 21 51 + 98754321 Kynea 287 4223 16639 + 4198399 Lehmer 15 51 85 + 99947761 Leyland 57 145 177 + 43050817 lonely 1343 19634 31429 + 47326799 Lucas 123 843 15127 + 87403803 lucky 15 21 25 + 9999997 Lynch-Bell 15 magic 15 34 65 + 93084991 magnanimous 14 21 25 + 91959398 metadrome 14 15 25 + 12345789 modest 26 39 46 + 99989999 Moran 21 111 133 + 99993563 Motzkin 21 51 323 + 853467 narcissistic 371 407 24678051 nialpdrome 10 21 22 + 99999997 nonagonal 46 111 559 + 99268651 nude 15 22 33 + 77717171 oban 10 15 25 + 998 octagonal 21 65 133 + 98728033 odious 14 21 22 + 99999993 palindromic 22 33 55 + 99977999 pancake 22 46 106 + 99976871 panconsummate 10 14 15 + 3097 pandigital 15 21 141 + 16434817 partition 15 22 77 + 82010177 pentagonal 22 35 51 + 99898801 pernicious 10 14 21 + 9999998 Perrin 10 22 39 + 49405543 plaindrome 14 15 22 + 88899999 Poulet 341 1387 2047 + 99945007 power 25 49 121 + 99460729 powerful 25 49 121 + 99460729 primeval 1037 1079 1379 + 12345679 Proth 25 33 49 + 99991553 rare 65 repdigit 22 33 55 + 1111111 repfigit 14 1537 7647 + 44121607 repunit 15 21 57 + 99910021 Rhonda 54479 245579 256973 + 98655551 Ruth-Aaron 15 25 49 + 98793593 Sastry 183 self 86 121 143 + 99999917 self-describing 22 10183133 10183331 + 42292429 sliding 25 65 133 + 10000001 Smith 22 58 85 + 99999706 square 25 49 121 + 99460729 star 121 253 793 + 99951853 straight-line 111 123 159 + 7654321 strobogrammatic 69 111 689 + 99911666 subfactorial 265 super Niven 10 super-d 69 106 119 + 9999919 tetrahedral 10 35 tetranacci 15 10671 20569 + 14564533 triangular 10 15 21 + 99214741 tribonacci 274 10609 trimorphic 25 49 51 + 500001 uban 10 15 21 + 99000097 Ulam 26 38 57 + 10000241 undulating 121 141 161 + 9797979 unprimeable 206 326 514 + 9999965 untouchable 146 206 262 + 999806 upside-down 46 55 82 + 99928111 vampire 117067 124483 146137 + 79168819 wasteful 22 26 33 + 9999998 Wieferich 3279 7651 10533 + 3837523 Woodall 159 323 895 + 90699263 Zuckerman 15 111 115 + 1111117 zygodrome 22 33 55 + 99999911