ɪn klˈæsɪkəl mᵻkˈænɪks, bˈɜːtɹændz θˈiəɹəm stˈeɪts ðæt ɐmˌʌŋ sˈɛntɹəlfˈoːɹs pətˈɛnʃəlz wɪð bˈaʊnd ˈɔːɹbɪts, ðɛɹˌɑːɹ ˈoʊnli tˈuː tˈaɪps ʌv sˈɛntɹəlfˈoːɹs ɹˈeɪdɪəl skˈeɪlɚ pətˈɛnʃəlz wɪððə pɹˈɑːpɚɾi ðæt ˈɔːl bˈaʊnd ˈɔːɹbɪts ɑːɹ ˈɔːlsoʊ klˈoʊzd ˈɔːɹbɪts.
In classical mechanics, Bertrand's theorem states that among central-force potentials with bound orbits, there are only two types of central-force (radial) scalar potentials with the property that all bound orbits are also closed orbits.