the lengthened of the aṅga (stem) of tan is optional before the Desiderative san when it does not take the augment iṭ,
Thus तितांसति or तितंसति ॥ But in तितनिषति no alternative is allowed, as इट् is added to सन् by 7.2.49 Vartika: optionally.,
तनोतेर् विभाषा
Adhyāya 6 · Pāda 4 · Rule 17