Bolesław the Forgotten (Polish: Bolesław Zapomniany) or the Cruel (Bolesław Okrutny; before 1016 – 1038/39) was a semi-legendary Duke of Poland of the Piast dynasty from 1034 until his death in 1038 or 1039. He was allegedly the first-born son of Mieszko II Lambert.

http://genealogy.euweb.cz/piast/piast1.html

The Piast family

Ziemomysl, Ct of Poland (922-962), +ca 962/963 (possibly a son of Leszek); He had issue:* ...

• A3. Mieszko I, baptized in 966, Ct of Poland (962-993), *922, +Poznan 25.5.992, bur St.Peter, Poznan; 1m: NN; 2m: 965 Dobrava of Bohemia (*ca 937 +977); 3m: 979/980 Oda (+1023) dau.of Gf Dietrich von Haldensleben, Mgve of the Nordmark
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  • B2. [2m.] Boleslaw I "Chrobry" "the Brave", Ct of Poland (993-1024), became King of Poland (1024-25), Duke of Bohemia (1003-04), *967, +17.6.1025; 1m: 984 (div 985/6) N, a dau.of Mgve Rikdag of Meissen; 2m: 985 (div 987) Judith of Hungary; 3m: 987 Emnilde (+1017) dau.of the Sorb chieftain Dobromir; 4m: Czicania 3.2.1018 Oda (+1025) dau.of Mgve Ekkehard of Meissen
    • ...
    • C4. [3m.] King Mieszko II Lambert of Poland (1025-34), *990, +murdered 10.5.1034; m.Merseburg ca 1013 Richeza of Lorraine (*before 1000 +21.3.1063)
      • D1. Kazimierz I Karol, Ct of Poland (1039-58), *25.7.1016, +28.11.1058; m.1041/42 Dobronega of Kiev (+1087)
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      • D2. Ryksa, *1015, +1052; m.1039/42 King Bela I of Hungary (*1016 +1063)
      • D3. Gertruda, *1025, +Kiev 4.1.1108; m.ca 1043 Great Pr Izyaslav of Kiev (*1024 +1078)
    • ...