| 横から失礼します。
f(k) = mならばf(f(k)) = f(m) = 3kです。 よってf(f(m)) = f(3k) = 3m = 3f(k)です。
表を作ってみると、既に一部keiさんが述べられているように以下のようになります。
f(1) = 2, f(2) = 3, f(3) = 6, f(4) = 7, f(5) = 8, f(6) = 9, f(7) = 12, f(8) = 15, f(9) = 18, f(10) = 19, f(11) = 20, f(12) = 21, f(13) = 22, f(14) = 23, f(15) = 24, f(16) = 25, f(17) = 26, f(18) = 27, f(19) = 30, f(20) = 33, f(21) = 36, f(22) = 39, f(23) = 42, f(24) = 45, f(25) = 48, f(26) = 51, f(27) = 54, f(28) = 55, f(29) = 56, f(30) = 57, f(31) = 58, f(32) = 59, f(33) = 60, f(34) = 61, f(35) = 62, f(36) = 63, f(37) = 64, f(38) = 65, f(39) = 66, f(40) = 67, f(41) = 68, f(42) = 69, f(43) = 70, f(44) = 71, f(45) = 72, f(46) = 73, f(47) = 74, f(48) = 75, f(49) = 76, f(50) = 77, f(51) = 78, f(52) = 79, f(53) = 80, f(54) = 81, f(55) = 84, f(56) = 87, f(57) = 90, f(58) = 93, f(59) = 96, f(60) = 99, f(61) = 102, f(62) = 105, f(63) = 108, f(64) = 111, f(65) = 114, f(66) = 117, f(67) = 120, f(68) = 123, f(69) = 126, f(70) = 129, f(71) = 132, f(72) = 135, f(73) = 138, f(74) = 141, f(75) = 144, f(76) = 147, f(77) = 150, f(78) = 153, f(79) = 156, f(80) = 159, f(81) = 162, f(82) = 163, f(83) = 164, f(84) = 165, f(85) = 166, f(86) = 167, f(87) = 168, f(88) = 169, f(89) = 170, f(90) = 171, f(91) = 172, f(92) = 173, f(93) = 174, f(94) = 175, f(95) = 176, f(96) = 177, f(97) = 178, f(98) = 179, f(99) = 180, f(100) = 181, f(102) = 182, f(102) = 183, f(103) = 184, f(104) = 185, f(105) = 186, f(106) = 187, f(107) = 188, f(108) = 189, f(109) = 190, f(110) = 192, f(111) = 192, f(112) = 193, f(113) = 194, f(114) = 195, f(115) = 196, f(116) = 197, f(117) = 198, f(118) = 199, f(119) = 200, f(120) = 201, f(121) = 202, f(122) = 203, f(123) = 204, f(124) = 205, f(125) = 206, f(126) = 207, f(127) = 208, f(128) = 209, f(129) = 210, f(130) = 211, f(131) = 212, f(132) = 213, f(133) = 214, f(134) = 215, f(135) = 216, f(136) = 217, f(137) = 218, f(138) = 219, f(139) = 220, f(140) = 221, f(141) = 222, f(142) = 223, f(143) = 224, f(144) = 225
f(2007) = f(3^2*223) = 3^2*f(223)ですが、 上の表からf(142) = 223ですから、f(223) = 3*142 = 426です。 よってf(2007) = 3^2*426 = 3834です。
# 上記表の作成にはかなり時間がかかるので、 # 入試の問題だとするともっと上手な計算方法があるのかもしれません。
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