You subtract 7 from the current one to get the next.
f(1) = 13,
f(n) = f(n-1) - 7
Some numbers in the sequence:
f(1)=13,
f(2)=6,
f(3)=-1,
f(4)=-8,
f(5)=-15,
f(6)=-22,
f(7)=-29,
f(8)=-36,
f(9)=-43,
f(10)=-50,
f(11)=-57,
f(12)=-64,
f(13)=-71,
f(14)=-78,
f(15)=-85,
f(16)=-92,
f(17)=-99,
f(18)=-106,
f(19)=-113,
f(20)=-120,
f(21)=-127,
f(22)=-134,
f(23)=-141,
f(24)=-148,
f(25)=-155,
f(26)=-162,
f(27)=-169,
f(28)=-176,
f(29)=-183,
f(30)=-190,
f(31)=-197,
f(32)=-204,
f(33)=-211,
f(34)=-218,
f(35)=-225,
f(36)=-232,
f(37)=-239,
f(38)=-246,
f(39)=-253,
f(40)=-260,
f(41)=-267,
f(42)=-274,
f(43)=-281,
f(44)=-288,
f(45)=-295,
f(46)=-302,
f(47)=-309,
f(48)=-316,
f(49)=-323,
f(50)=-330,
f(51)=-337,
f(52)=-344,
f(53)=-351,
f(54)=-358,
f(55)=-365,
f(56)=-372,
f(57)=-379,
f(58)=-386,
f(59)=-393,
f(60)=-400,
f(61)=-407,
f(62)=-414,
f(63)=-421,
f(64)=-428,
f(65)=-435,
f(66)=-442,
f(67)=-449,
f(68)=-456,
f(69)=-463,
f(70)=-470,
f(71)=-477,
f(72)=-484,
f(73)=-491,
f(74)=-498,
f(75)=-505,
f(76)=-512,
f(77)=-519,
f(78)=-526,
f(79)=-533,
f(80)=-540,
f(81)=-547,
f(82)=-554,
f(83)=-561,
f(84)=-568,
f(85)=-575,
f(86)=-582,
f(87)=-589,
f(88)=-596,
f(89)=-603,
f(90)=-610,
f(91)=-617,
f(92)=-624,
f(93)=-631,
f(94)=-638,
f(95)=-645,
f(96)=-652,
f(97)=-659,
f(98)=-666,
f(99)=-673,
