Erik Berglund 0004 Jiaojiao Zhang Mikael Johansson 0001 Soft quasi-Newton: guaranteed positive definiteness by relaxing the secant constraint. 783-812 2025 July 40 Optim. Methods Softw. 4 https://doi.org/10.1080/10556788.2025.2475406 db/journals/oms/oms40.html#BerglundZJ25 streams/journals/oms